Well, you do not indicate your budget, but here are a variety recommendations by price point in South Florida ($=<250, $$=250-400, $$$=400+):

1. Casa Morada, Islamorada. A funky chic resort, no beach though. But the best Keys beaches are drivable from the resort ($$)
2. Little Palm Island, 28 miles from Key West on its own island ($$$ and then some). One of most famous resorts in Florida, very Caribbean in feel.
3. Simonton Court, Key West ($ to $$ depending on room) A cute Key West B&B right in old town.
4. Kon Tiki Resort, Islamorada ($) Hokey name but has a very good reputation amongst locals
5. Ritz Carlton, Key Biscayne ($$-$$$) Not quite as trendy and therefore a much better deal than its sister property in South Beach. On a beautiful stretch of beach too. And you can easily drive to South Beach for dinner.
6. The Pillars on New River Sound, Fort Lauderdale ($$), probably as close to a boutique B&B as you will find in the area. No beach, but it is within walking distance
7. Il Lugano, Fort Lauderdale ($$-$$$) Gorgeous new boutique hotel that probably has some good deals since it is not on the beach, but has a good intracoastal location an easy drive from the beach. Houses the new Todd English restaurant, the only one in SF.

BTW, weather here can be a bit hit or miss in January. Might be 80 degrees, but then again only 60's. The Keys are the best bet for warm weather. I would not recommend central or northern FLA in Jan because it can get down right cold.