Do you travel with a clean towel as in the Hitch-hiker's Guide? If so, check your bags and take your towel and swin trunks on the local train to Bad Homburg on the north side of Frankfurt. You can walk from the train station or take a cab to the Tanus Therma -- this is a large, excellent spa and an extremely relaxing place to recover between flights. Upstairs it's FKK for the less inhibited.