From our winter "base camp" at Knight's Key campground in Marathon, we can walk or bike the Old Seven Mile bridge span that leads 2 miles out to historic Pigeon Key. The other night on a sunset bike ride I saw people of all ages and abilities: walkers, joggers, bikers, skateboarders, and a woman in a wheelchair. Dogs are welcome too (just please, pick up after them).
It's really become an important means of recreation for many people on an island where the highway, US 1, is pretty dangerous for pedestrians and bikers. From it height you can watch for dolphins, sea turtles and sharks in the water below.
It's also historic, one of the original bridges Henry Flagler had built back in 1912 as part of the railroad from Miami to Key West, thus connecting that island nation to the mainland for the first time in history.
Long story short, it's falling apart, and the DOT, which owns it, really isn't in the business of fixing bridges that don't carry vehicles anymore. They would rather close it for safety reasons.
Enter the Friends of Old Seven, a volunteer group that is raising funds (DOT will match them), to fix the bridge. The volunteers are out there everyday, with an information table set up at the entrance to the bridge. For a donation you can get a T-shirt, tote bag or a bumper sticker for you bike like I did!