Helen, to ease your mind, the airport is very close to Frankfort. Doug and I stayed even further out than Frankfort in a suburb last year (my corporate headquarters is there) and we spent a day in Frankfurt. You can have a very nice time without rushing or stressing with the time you have. Frankfort is a compact city that is walkable. The museums are located all together and you can do a very nice several hour walking tour of their old town. With both museums and a tour, a nice lunch and dinner, you will have a nice day. Have a great time.