If you're going to Key West you have to see the Hemingway House. Also have to drink at Hemingway's old watering hole Captain Tony's.
There's a sweet glass bottom boat ride in key largo I believe. That was one of my favorite experiences. In key west there are a few companies that's do like all day excursions out to a reef when I went it was $50/person. Provided snorkel gear. They also have wave runners and tubing they do out there and para sailing for an extra $50/person.