As you are flying from Winterhaven, have you considered a couple of days float plane flying from Jack Brown's? Great fun and you get a float rating at the end. Not much use in the UK, though!
Also you could take a trip to Everglades City and do the air boats ride around the everglades. Worth staying overnight for dinner, especially if you like eating crab.
Also recommend Cedar Key, as above. If its a hazy day with no clear horizon, make sure your instrument flying is OK. There's a plaque on the airfield commemorating two local pilots who lost their lives because they lost the horizon over the sea (on take off I think).
Finally, a scenic day flying down the keys to Key West and stopping off at Marathon. It's quite boring by car, but magic by plane.
Do you know that you need to convert your licence to a US airmans certificate? No big deal, but it is a must. Most rental FBOs will be able to arrange this.
Whatever you decide, be safe, enjoy...
Regards.