E.M.
In case you'd still consider it--if you do more research you'll find all the violence in Mexico is really revolving around specific border areas and inland. The resort areas have not been targeted or have seen any rise in violence. We went to Playa del Carmen last May and it was wonderful!
But if that's no longer an option, I'd second the idea of Key West and Puerto Rico. Both places that should have nice weather and a great night life. (If you're looking for lots of long stretches of beach though, Key West doesn't have them. More poolside fun.)