Be aware of issues regarding traveling to Cuba by US nationals due to the imposed travel regulations to US citizens by the US government. Technically, it is our understanding the regulations are not against traveling itself but spending or trading in Cuba which in any case, seems like pretty much the same thing.
However, we also know that US travelers still come to Cuba regularly using third party countries like Canada & Mexico.
In counterpart, Cuba will not stamp your passport; and instead uses flying visas or "tourist cards" Airlines usually can assist in providing such tourist cards.
Please do note that sanctions can be in place by US authorities of up 250 000 US dollar fees and that traveling to Cuba as a US citizen is solely your responsibility.
From December 2014 an ease of relations between Cuba and the US has made traveling to Cuba a bit more flexible, expanding the authorized people who can actually travel to Cuba. Authorizations are issued under several licenses, 12 categories in total
You can find plenty US official information in the following links: