Do a pub crawl. Hog's Breath Saloon, Margaritaville and Sloppy Joe's are a good start, or you can pay for a guided tour- The Duval Crawl
Last time there we noticed the Tall Ship Harbor cruises,( Schooners) wish we'd seen it before we ran out of time.
I would definitely take the Conch Train tour, you get a narrated tour and it gives you a good insight into the History of the island, you get to see quite a lot in an hour or so. They stop at the Southernmost point for photos.