I have driven the Overseas highway many times, and in an ordinary vehicle the views are nothing special. The bridges have high solid walls and the highway itself is disappointingly lined with Subways, MacDonalds and hotels so you could be almost anywhere. It is better in an SUV or higher vehicle and brilliant in an open-top car!
I'm not sure the Keys is the place for you, Jilly as it is very hot and humid, the beaches are tiny with soft sand you quickly sink into, so no chance of beach walks and hikes. The two things visitors go to the Keys for are the city of Key West, and the watersports opportunities. Key West is brilliant for a couple of days sightseeing, shopping, art galleries, maritime museums, historic homes and gardens. Watersports lovers will find superb snorkeling and diving opportunities, sport fishing charters, sunset boat cruises, glass bottom boat trips and Seadoo safaris are all available at Key West, and to a lesser degree at Marathon, halfway along the Overseas Highway.
For hikes, try Lake Okeechobee, or visit the Everglades National Park at Everglades City/Flamingo where there is camping, hiking trails and great ranger-led hikes and activities. Hope you have fun!