I have been to Cuba 4 times since 2008. While it is a beautiful, safe and cheap country to vacation in, there are several important things you should know when trying to enter or exit the country.
1. Don't mix up the two currencies of the country. There is the Convertible Peso (CUC) which is used by tourists and the Peso which is used by the locals. The Convertible Peso is worth 24 times more than the Peso so if you are unfamiliar with the currency make sure you only exchange your money at a certified currency exchanger at the airport or the hotel or you may accidentally overpay for the currency that locals use. There is no real advantage to exchanging at the hotel or airport. The airport may be slightly cheaper in that you save 50 cents for every 100 CUC purchased but I prefer to exchange at the hotel when I am settled and relaxed. Avoid bringing exclusively US currency along with you on the trip because Cuba charges you a 10% commission to exchange it for CUC in retaliation to the embargo.
2. On your flight to the country you will receive a Customs card with two identical sides to it. Fill out both sides accurately with the same information, namely your full name, citizenship and Passport number. If you make a mistake you will have to pay for a second card. As you enter the country the Customs Office will take the first half and stamp the second. As you leave the country they will take the second half and record how long you were in the country. Treat the second half of the card with the same level of care as you would with your passport. I never lost mine so I don't know what the consequences are if you do lose it, but I can imagine that it's probably not a situation that you want to put yourself in.
3. Make sure you have 25 Convertible Pesos for the Airport Tax when you leave the country. This tax is paid after you check in for your flight home but before you go through customs and security.
4. Do not pick a physical fight with a Cuban. Cubans are very laid back and friendly people and there are strict laws against them being violent with tourists so you will not have a problem unless you provoke something. If you get into a fight with another tourist, the police will detain you until your flight back home is ready. Not fun, but at least there are no long term consequences. But if you get into a fight with a local, you will be charged and will be at the mercy of the Cuban legal system.
5. Avoid buying fake Cuban cigars. I have seen them seized by the Cuban customs a couple of times already. To ensure you are not getting fakes only deal with the stores at your resort or the airport.
Refer to my other articles on travelling in Cuba if you are interested in information about:
General information on vacationing in Cuba
How to get the best deal for your Cuban trip
How to have a party vacation in Cuba
How to have a relaxing vacation in Cuba
Good Cuban products and souvenirs
Things to Donate when Vacationing in Cuba