You could take the train into Frankfurt itself if you want - the train goes from the airport train station to Frankfurt main station pretty frequently. The journey takes about 10-15 minutes. I had a 5 hour layover at FRA a couple of months ago and did this with no problem.
The only thing I would say is, if you do leave the airport, make sure you leave plenty of time on your return to pass through security as it can take a while.
I think there is a "spotters" terrace at FRA if you're into that kind of thing, but I think I heard somewhere it was getting closed down soon.