I'm attending a conference in the Ritz Carlton Key Biscayne at the end of Feb. The conf runs Mon noon - Tues evening, and I'm on a 4:40 PM flight back to Denver (the other flight choices either involved an EARLY flight and a connect, or a connect so that the travel time > the 4:30 direct flight).

Landing Sunday evening about 4:00 PM. I have Mon AM thru noon and most of the day Thursday. I'm staying at the Marriott Courtyard South Beach.

Any recommendations? I love food, culture, history, water, and am an outdoorsy type. Might be interested in poking around Little Havana (if such a thing exists and isn't the kind of tourist trap that "Little [ethnicity]" is in other cities). Maybe a cigar factory? I'd love to go spearfishing, but I don't think the logistics work out. Maybe kayak rental?

Also...how impractical is it to drop bags at the airport early in the morning on Thursday, then leave teh airport to do "whatever"? Or am I better leaving bags at teh front desk, doing my thing, then coming back? Is the beach OK or too cold?

If anyone can guess what conf I'm referring to and will be there, let's meet up! I'll buy the first drink.

Since you are staying at the Marriott Courtyard South Beach, you should check out Lincoln Road Mall. Nice stroll of shops, restaurants, and especially people watching.
Evenings are best.
http://www.lincolnroad.org/

I enjoyed the stroll on Lincoln Road. I was there Sunday afternoon, and they had what I'd characterize as a "high class flea market" that closed at about 6:30. I bought some cufflinks from an estate jewelry guy. I'd also recommend buying a cigar or a couple of drinks and people watching. There is a restaurant called "Yuca" that has an upstairs (not affiliated) that offers salsa dancing classes on Sun, Weds, and Fri. $29.00 for about 2 hours of instruction and an hour of practice.

Later in the evening, Ocean Drive is good for people watching/barhopping. There is a public beach as well.

Please note that taxis are EXPENSIVE ($5.00/mile; $43.00 from MIA to Key Biscayne or $33 from MIA-South Beach) and not all of them take plastic. If that's an issue, ask when you call a cab and they will get one for you. They have to run your card by hand, and call it in....takes about 5 minutes to resolve. $5.00 does not sound like much, but MIA is surprisingly spread out.