I hear fabulous things about Cuba, but as an American it is difficult to get there. Check Air Canada. I can't even find airfares even while using a Canadian airport.
As for other Caribbean Islands, the smaller ones can be expensive to get to, but reasonable once you are there. I have done most of them via cruise ships. I do know that it is possible to do round trip San Francisco to San Juan Puerto Rica for $400 US. Jamaica might be reasonable as well. If you are just looking for warm weather and beaches, you might consider Puerto Vallarta or Cancun. Bon voyage.