May be venturing to southern Florida for some fun in the sun and other 'indoor activities' with a guest.

Was thinking of flying into Miami and driving down to the Keys for 4 days. I don't need to go all the way to Key West. I'm looking for an island that has a nice beach (seclusion a plus  , maybe some snorkeling. Not a big party or night life scene. Can anyone offer some advice on islands I should look at? Thanks.

(And I love all the Tushie & Leg threads you guys have on the board here!)