Will you have a car? I road trip up the Rhine to Bonn/Cologne is nice. A couple small interesting towns along the way. Heidelberg is close and has a great castle. If you are going to hang out near FRA, I would suggest Mainz, a very short train trip.
Berlin is a great city. But it is quite a distance from Frankfurt and if you have to return to FRA it will take 2 days travel out of your trip.
Almost every town has a pedestrian road with shops/restaurants/bars. Always a good place to explore.