DH and I went about 10 years ago. I don't remember all of the details, but it is one of my favorite trips that we have taken. We spent a couple of days on Ft. Lauderdale visiting some friends, then drove to Key Largo and went snorkeling at Pennekamp State Park. We happened to be there right before a hurricane, so the waves were pretty rough and I struggled with the snorkeling, but it was enjoyable otherwise. We spent a night there (maybe two, I can't remember for sure), then kept driving. We spent an afternoon at Bahia Honda State Park. They have a pretty nice beach. Then we drove on to Key West. I think we spent two nights there. We walked around downtown, toured the lighthouse, went to the southernmost point, watched the sunset in Mallory Square. We had a trip planned to Dry Tortugas, but they had to reschedule it due to weather (I think) and we decided to skip it and just relax. Then we went to Miami for a couple of nights. South Beach was gorgeous and we mostly just hung out on the beach or at the hotel pool.
I can't remember the details of where we stayed, so I'm no help with that. I spent lots of time on TripAdvisor and got good recs there.
DS - Feb 2010
DD - May 2012