Here's a few things you may want to consider before you pack up:

Electricity

110V. You need an adaptor(which is different than a transformer...). Unless you stay in a luxury hotel (that may provide adapted plugs for Europeans), Cuban plugs are wobbly. So make sure this is charging! And don't forget a multi plug, you may find that only one of the plugs in your room is working…

Money and sense

The country has two parallel currencies: CUP (Cuban pesos, for Cubans only), and CUC (convertible pesos, for naïve tourists that pay 20 times more than locals). If you find yourself in possession of CUP you have probably been cheated: some denominations look confusingly the same but beware: 1 CUC=25CUP!
Do NOT bring USD! They are submitted to an enmity (!) tax, and therefore you may have a hard time paying with them. Euros are OK, but the king of the foreign currencies is definitely the Canadian dollar, at the incredible rate of 1,40 CUC for 1 CAD (December 2016). Do not bring foreign coins, they are useless.
Cubans prefer CUC, called familiarly cucos.
You can change your money at the airport, it will only take you 2 hours to do so… There is (almost) no unofficial exchange market. Exchange rates barely vary, and unless you are VERY tight the  difference is barely noticeable. Some big hotels change money (usually a service limited to guests, but you can try) and there are no banks as you imagine them. You will have to change money in one of the numerous CADECAs (Casas De Cambio). Look for the loooong tourist queue, and it will be either a Cadeca, an Etecsa internet-card selling point, or Coppelia, the famously notorious (!)  ice cream parlour (see below).
As for credit cards, leave your AMEX home (almost an insult even showing it in Cuba), and just take Visa and Mastercard, although you may never be able to use them.
Take tons of cash. Small currencies! (the monthly salary of a Cuban is around 50-60 euros, so handing a 5 euro bill may appear normal to you but can be complicated to change).

Telephone

Your mobile phone will work OK, but don't even think of using it  in Cuba. Even if they call you, roaming charges are extortionate. So just pray nothing wrong happens while you are there.
Cubans will eagerly let you use their landline. But calling in Cuba is, as all else, not a straightforward experience. You need to know if the other number is a mobile or a landline. Add a different prefix for each one. Follow instructions that routinely come in Spanish from the other end: "the number you are calling has changed. Please substitute six numbers by XXX" (not presicing the first or the last ones, before or after the prefix etc.) In this case, ask a young fellow, older Cubans may not be able to help you… Unfortunately, you will have to use the landline of your landlord (!) while in Cuba, since nothing can be arranged or confirmed if you don't call one or two days before. Even booking a restaurant, especially a good one (ie La Guarida) will ask you to ring twice or they will cancel your reservation.
If you travel independently, as I strongly recommend, all your reservations or almost will be made from your home country by telephone. Cuban hotels are not in Booking.com… So Skype you will, and if you manage to form the right number, Cubans will usually only ask for your first name and this is it! You are ready to depart to Cuba with merely an oral  promise for all your bookings! Many foreigners profit from this system, booking without appearing, and you need therefore to reconfirm some days before, no matter how frustrating that is. This goes especially for casas particulares, but also restaurants and long-distance taxis! Speaking Spanish will help immensely.

Internet

Be smart enough to see your stay as an opportunity to detox. Which you will end up doing anyhow, since connections are extremely difficult, unless you stay at a big luxury hotel that will still deliver a shitty WiFi. Otherwise, if you can't resist this selfie showing off to your Friends what a superb time you are having, there are now wifi hotspots in a few places in every city: you can check the Etecsa site, although you will realise where exactly when you see hundreds of people looking at their phones, presenting a surreal sight in the night, with their faces illuminated by the mobile screen in the dark! You will need to wait a huge cola (queue) to buy an ETECSA card that will provide officially 1 hour of shitty, nerve-wrecking connection  during one month for 1 CUC (2CUC or more if you buy them from a local, usually nearby the selling points, but without queueing up). Speed is extremely slow, and a connection is hard to get by (be there early in the morning), but this may be your only chance  to send an email, a photo or a message. Forget Facetiming or Skyping. Bring a picture of your beloved ones instead and use the occasion to actually miss them!
In some other non official places you will see people surfing in smaller numbers, mostly locals. Sit somewhere near, pretend you are trying to use your mobile and you will promptly be approached by a local offering a connection, for one CUC typically. It will of course be a broken-in public network, but frankly no one seems to care.

Post

No one would bother under normal circumstances, but since this is Cuba you need to know that letters take typically at least a month to get home. So send these Christmas postcards before you leave!

Water

You will have to buy it, a total rip-off. 1 euro for a small bottle, or more. Exploiting tourists at a max, you will barely see any 1,5 l bottles.

Medicine

BYO (bring your own). No rare diseases, no Zica. But no pharmacies either if you need them (they are almost without exception empty). Head to Miramar (the "posh" district of Havana) if you badly need a doctor, or -even better- head home, with a good repatriation policy if anything goes seriously wrong. Pray if shit happens while you are anywhere outside Havana…

Transport

Planes
Coming from Europe, you probably flew in with Iberia or Air France, both as shitty as Ryanair, but in a transatlantic scale. It would do you no harm to bring your own sandwich or chocolate for the trip on board. I mean it...
Once there (José Marti international airport), a taxi to Havana will set you off 25 cucos, and this will almost definitely be the one and only time you will pay a fixed price for a taxi during your whole trip.

Taxis
Totally, totally unreliable and very expensive. Taxi drivers have gone berserk, asking you 5, 6 or 10 euros for two kilometres (engaging in sometimes funny, sometimes nerve breaking conversations with drivers, one told me once that his car is very old and burns 1 litre per kilometre!). Moreover, never ever rely on their word for bringing you back from where they left you (ie the beach, the bush or the middle of nowhere). They will not show up and you may get yourself in trouble getting back, depending where you are (my daughter was eaten alive by sandflies, attacking typically right after sunset, when we were stood up by a taxi at the beach: next time we were stood up again, but we had long trousers and mosquito repellent...).
When you stop a taxi you need to suggest a price for your destination (ie "Hotel Nacional, 6 cucos, OK"?) This will not necessarily sharpen your linguistic skills but if you don't present yourself this way, the sky is the limit… Don't stop a taxi with an expensive camera hanging from your shoulders, this is the signature that you are rich and willing to pay whatever ridiculously high fare. And don't listen to your guide: flagging down a taxi and trying to negotiate your fare beforehand is only showing you have not a clue how much you should pay.

Coco taxis
The cute, photogenic motorcycles converted into lemon-yellow taxis with a fiber-glass coconut shell are equally satanically expensive. Walking will not only save you money but possibly also your life: coco taxis are notoriously unsafe. Conceived originally by Fidel as a job opportunity for women, they are nowadays not driven anymore by beautiful Cuban ladies, looking for a fare and possibly a romance, but by macho Cuban men  that race like there's no tomorrow (…).

Bicycle taxis
My preferred way of getting from A to B in Cuba. The problem is though, you can't reach very far and you can barely start a conversation with the drivers, as they struggle to pedal through crowds and very difficult roads. On the other hand, to keep your ears busy bicycle taxis usually come together with a ghetto blaster and tons of Regueton music, whereby all songs are written on a 3 note pentagram, making them resemble  more a noise than a melody.

Buses
Avoid the juaguas (articulated buses, or camelos=camels, because of their humps). As a tourist, you will be prompted to take a Viazul or Transtur bus (as I said, freezing temperatures inside). It's not easy getting a ticket, and even if you do, good luck with the timetables… Buses are routinely (very) delayed or cancelled. On the other hand, they are very cheap.

Bikes
The bike you may hire will resemble and feel more like riding an iron hanger on wheels. Some guides and sites will encourage you to bring your own bike and leave it there "to help the transport crisis" in Cuba. What a practical idea!
Although we met some people with decent rented bicycles, some running across Cuba for weeks, the ones we got were horrid. A big, big frustration for me.

Cars
Renting a car is expensive and risky. The risk being theft and the state the roads are in. Never drive at night. You are NOT home, don't take that risk, especially in the dark: Cuban laws are very heavy on foreigners in case of accident! People, horses, horse carriages cross highways all the time, and potholes are enormous. There is no light anywhere in the night, and signalling is very bad. Fuel is very expensive.
Hiring a taxi for big distances is comparatively to city taxis not very expensive, but once again, that depends on how shiny is your camera… Safety is a huge issue. Pollution too (when our bus did not show up, we rode Cienfuegos to Trinidad in a taxi that somehow diverted all fumes in the passenger compartment, an unending, sickening full hour of driving).

Trains
Ha! Unless you have eternity at your disposal, and solid nerves, you should opt for another means of transport. If only teleportation worked... We did manage to take the train from Trinidad to La valle de los ingénios, and are still coughing fumes out, but ohterwise it was a very colorful experience. My daughter had an outing proposal from the train barman (!), and I think a nice lady waved at me as we were passing through a village, but it may have been the fumes...

But there are some boats you may take (ie crossing the port of Havana to get to La Regla and take the Hershey train, for example, or the sunset tour boat from Cienfuegos), and of course horse carriages, probably the cheapest but almost exclusively Cuban way to commute.

As a general rule, during your transports in Cuba you will have to get used to two things: deep-freezing air conditioners and  compulsory music! Btw, the same goes for taxis, buses or whatever means of transport in Cuba: cars routinely lack safety belts, but they all feature  superb sound systems, with many Russian Ladas and  almendrones (as Cubans like to call their American old timers: big almonds, because of their shape…) featuring  a small TV playing non-stop video regueton clips (Shaky shaky was so, so contaminating and I am ashamed to admit I sometimes still play it in my car...)

See this article for more on Cuban transport.