Upper Keys Beaches are far and few in between. I think Anne's Beach at mile marker 77 oceanside is the only natural beach this side of Marathon. The water there is quite shallow, but you can wade out, or float around with a mask and snorkel for a close-up view of the prop-scarred oceanfront grass flats. I find it interesting, but it's unlikely that your average 16 year old would be impressed.
Many resorts in the Upper and Middle Keys feature manmade beaches amongst their amenities, but they are, in my experience, little more than sand that has been trucked in and dumped along the waterfront where the mangroves once grew. Holiday Isle in Islamorada has a big manmade beach and swimming area, along with lots of aquatic activities to choose from, including sunbathing, diving, snorkeling, fishing, kiteboarding, parasailing, boat and PWC rentals, etc. There are several bars and restaurants onsite, and live music to boot.
We (BlueWater Divers) are located at Key Largo Resorts, which features three resort hotels, with several swimming pools, hot tubs, bars and restaurants, and water related activities, including sailing, snorkeling, diving, fishing, parasailing, boat rentals, and glass bottom boat tours, but alas, no beaches!
Topside activities are numerous. There's the previously mentioned Wild Bird Center, as well as the new History of Scuba Diving Museum. Shopping oportunities abound. There are also several human-dolphin interaction programs, and the nation's second oldest marine park, Theater of the Sea. The Turtle Hospital in Marathon is now open to the public. Everglades National Park, Biscayne National Park, Bill Baggs State Recreation Area (on Key Biscayne), John Pennekamp State Park, and Bahia Honda State Park are all within easy reach of the Upper Keys, and well worth the effort.
I hope this helps. Let me know if you need any additional (or more specific) information.