Plan to bring lots of cash. Lots of it. American banks are still not synced up to the Cuban banking system so this essentially means no ATM access for U.S. citizens (reports of Master-Card accepting ATMs in Havana are vastly overstated). Most businesses will not accept credit cards. Don’t let this point stress you. Just make a budget and plan to bring 30-40% more than you think you’ll need. Unplanned expenses always come up and you don’t want to be pinching pennies on your vacation. As a thank you for your visit, the Cuban government extracts a $25 fee from each visitor upon exit so don’t forget to set at least that much aside on the day you plan to leave Cuban soil (side note, on most charter flights this $25 is now collected upon departure from the U.S.).
Some people get caught up in what kind of money is best to bring. Keep things simple. Just bring U.S. dollars or Euros. There are plenty of money exchanges (Casas de Cambios) and hotels in Havana who will give you the best exchange rate at the time. Don’t ever exchange money on the street. Keep it official and you won’t get ripped off.