Having just returned from Havana (a stop on the Norwegian Sun cruise line) I wanted to share some insights about what to expect.
1. you do NOT need to book a tour from the Norwegian Sun cruise ship. In fact you will be ripped off! They make it seem like you must book with them or get stuck on the ship. Not so. We lucked out when a friend recommended a US-based tour operator (Vaya Travels - http://www.vayatravelstocuba.com/) who have lots of connections in Cuba. Our tour was very personal, it included a 3 hour tour in a vintage car, guided tout of the Art museum, walk around old town, lunch, dinner, and a night club where Cuban singer Haila was performing. It was brilliant, and cost less than one of the Norwegian tours on its own (while it was actually about 5 of them all stitched together!).
2. take currency other than US dollars (we took Euros) since you avoid the tax they put on USA $ (you can order Euros from any USA bank)
3. If you have a non-USA ATM card take it (you can use it, you cannot use a USA ATM card)
4. Give your left-over CUC money (Cuban money for tourists) to your guide - s/he will appreciate it more than you will if you were to change it back! And always tip them and invite them to join you for drinks, lunch or dinner
5. Get small change (coins) asap so you can give them to the toilet attendants (they have attendants at the art museum and at night clubs), and also to people who check your coats/bags (art museum) and people on the street who pose for a photo, etc.
6. Pack mosquito repellent, wipes and toilet paper - these you will need! And bottled water.
7. Only drink bottled water (unless you're used to the Cuban water then you're ok)
8. Ice is ok in the good restaurants (you can ask if it is filtered), and the rum is divine!
9. It takes time to get off the cruise ship (if you arrive by ship) so get a ticket to get off the boat as soon as possible. Norwegian starts to give out tickets about an hour before the first person is allowed of. Our group was number 3 and we were able to meet our guide an hour after the boat docked. Later groups (there are over 20 groups typically) will mean a much later start.
10. Don't turn your cell phone to roaming or take off airplane mode until you LEAVE the ship - the ship's internet will kick in and you will get a charge on your room without realizing it!
I hope this helps - Have a great trip!