I second Cozumel, particularly in August/September (low season, hardly any tourists) and don't forget to include a day trip or 2 to dive/snorkel the cenotes in the Yucatan peninsula and visit the pyramids. I usually book my packages thru Felix at Bay Adventures:
http://www.bayadventures.com/
Also Bahamas (New Providence) land based, through Stuart Cove's:
http://www.stuartcove.com/
I loved staying at Orange Hill Inn (economical but nice)
Northern Bahamian Islands through Blackbeard's liveaboard (departs Grand Bahamas):
http://www.blackbeardscruises.com/
be forewarned Blackbeard's dirt cheap, living/diving conditions are spartan (aka camping at sea).
Have a great trip!