You would even have enough time to go in town. It's a 10 minutes train ride to the city which will only cost you less than 3 EUR. Just make sure to be back in time to clear security and passport control.
If you prefer organized tours, Frankfurt airport is offering tours as well, but I guess only Frankfurt would due since you have time contraints: http://www.frankfurt-airport.com/con...rport/en/bookings/sightseeing.html
Otherwise, HT already said what you could do if you prefer remain near the airport. If you're into spotting and if weather is good, you may also go to the popular spotting points either west of the airport, next to the Startbahn west, or East, next to the A5. You get great views there, although it can get quite full on weekends if weather is nice. And it's free and will cost you nothing. However there is quite some walk (or a short taxi ride) involved. More infos at fraspotters.de or just google.
You may also make an airport sightseeing tour: http://www.frankfurt-airport.com/con...joy0/airport_sightseeingtours.html
If you want to buy something to eat or drink and not spend a fortune, head down to the TEGUT supermarket, which is under Terminal 1, area C, next to Hooters... They also have some hot german fast food stuff.
There is plenty to do in 6 hours, have fun!