1)Snorkeling on Loo Key Reef!
2)Go to the pier at the end of Duval St. and watch the sunset and all the crazy acts. Some are actually pretty good.
3)stop in Captain Tony's for a brew. Not all commercial like Sloppy Joes(Very rustic and old)
4)For some excellent seafood make your way up to the 7-Mile Grill just where the bridge meets the shore on the south side of Marathon Key.
5) On your way to Marathon have a swim Bahia Honda beach. On your way back to Key west stop at Big Pine Key and do some site seeing: Key Deer, the Blue Hole (can see aligators), walk across "No-Name bridge" to "No Name Key" then have a brew at "NO Name Pub". From the bridge you can see rays, sharks, tarpon, etc... swim under the bridge. No Name pub is a must!
6) If I think of some more I'll post 'em, or if you want to know more you can just PM me and we can talk if you would like. Me parents own a home on Big Pine. They are down there right now.
http://www.floridakeys.com/lowerkeys/bahiahonda.htm
I used to jump off Bahia Honda Bridge and let the curreent pull me around to beach on the back side of the island until I saw a picture of who lives and hunts under that bridge..."BIG MO" Just ask who Big Mo is while your down there.Hahaha...You can see on that link page where I would jump off; it's where the portion of bridge is missing closest to the island.
I have the correct link up now. Oops