Just a few ideas...
Get a bucket of fish and let the Tarpon come grab them right out of your hand. So much fun and a place to have lunch too.
A beautiful place to see! Largest beach in the Keys and a great day to be had. Pack a picnic and a towel
An amazing and peacful drive that connects all the keys. Amazing places to stop before and after the bridge. Get extra fancy and try biking or running the bridge.
Take a full day or half day boat charter and try your luck on catching some tuna! Most resteraunts in Islamorada will actually cook you your catch for dinner!
See all the fishies of the Keys in the middle of the ocean at one of the most beautiful reefs in the sea.
Island hop, swim and relax from key to key.