bluefish's recommendations are good... I'll just add in the islands off of Los Angeles: Channel Islands National Park (Santa Cruz or Santa Rosa) or if you want a touch of civilization then Catalina Island (not part of the NP but geographically part of the Channel Islands). Winter is my favorite time to go. Great weather (though a chance of rain) and not very many people.

I haven't backpacked out side the US but Cuba, Puerto Rico, or Hawaii sound like a lot of fun!