On the north side of the Seven Mile bridge, there is a shack on the right hand side as you are traveling south. It used to be called the Seven Mile Grill. It looks like you shouldn't touch anything let alone eat there. Stop in and get a piece of key lime pie. It will knock your socks off. That will be the find of the trip regardless.
As far as detecting, there is a state beach down around the middle of the keys that gets a ton of visitors. I know when we used to go (about 10 years ago) the gates would close around 11 AM because it was "full" so get there early. I don't know if you are allowed to detect, but for volume of traffic it definitely has got to be top two in the keys. Key West beach also sees a lot of people but it is a haul. You could check out the shipwreck museum if you made it to Key West though.
Good luck and bring me a slice of that pie please.
-Fathead