The most "Touristy" island would be Key West, Lots of sights to see as well a lot of nightlife. A good charter fleet and dive operations there too. Marathon and Islamarada have charter and dive boat operations. Key Largo has lots of dive boats that go to John Pennycamp Sanctuary which has suppurb diving. We go every year to Tarpon fish and we hide away on Big Pine Key. We bring our own boat and stay away from most places except The No Name Pub for dinner a couple nights. We mainly cook at the house and chill out.
If you have never been down there I would recommend stopping at Robbie's and feeding the Tarpon, Pizza and beers one night at The No Name Pub on Big Pine, a fine dinner in Flip Flops at the Square Grouper on Cudjo Key and of course one night of drinking on Duval in Key West.
This is inside the No Name Pub. Yes all $ bills.