The Overseas Highway is the spine of the Florida Keys, carrying US-1 over a long sequence of islands and open-water bridges between the mainland and Key West. It is beautiful, but not in a fast-road way. The pleasure is in the glimpses of turquoise flats, old bridge sections, pelicans on railings, and sudden little settlements where you stop for fuel, coffee, or fish sandwiches.
Plan the drive as part of the trip, not just the route to the trip. Traffic can crawl on weekends, and accidents can close long stretches because there are few alternatives. Leave early, keep water in the car, and build in time for state parks and roadside pull-offs.