In chronological order, and because there is no way I could pick just one;
1. Visiting the Che mausoleum at Santa Clara
2. Drinking with the locals in Sancti Spiritus
3. Hiking to Fidel's camp in the Sierra Maestra followed by a well earned beer and a whole hog roast in Santo Domingo
4. A night on the town at Casa de la Musica in Trinidad
5. Swimming in the waterfall at El Cubano outside Trinidad
6. Driving to a restaurant in a 1952 Plymouth
1. Don't bank on hot water :-)
2. Cuban hotels are characterful and generally great, but they aren't European hotels
3. Don't worry about the food, it's pretty good unless you're a vegan...
4. Try and change up some notes for coins for toilets etc
5. Learn as much Spanish as possible before you go, it will make your life a lot easier.
6. If you want to go on any of the optional excursions be prepared to push a little as they're not always promoted vigorously....(which is quite nice until you want to do one....)