Actually, there's quite a lot of interest in Frankfurt if you're prepared to look under the surface. You could hang out in Sachsenhausen which is quite a lively and pleasant district. And there's a good number of museums, if you're into that.

Not far away (a quick S-Bahn ride) is my personal favourite, Mainz, where you could scrutinize the Gutenberg bibles, visit a series of small but amazing churches (one has windows created by Chagall) as well as the cathedral (Dom) and walk along the Rhine waterfront. You could get lost in the old town among timber framed buildings and dance on the tables in breweries and Weingut establishments. The nearby town of Wiesbaden (just a few minutes with S-Bahn train) is really pleasant too.

Or you could hop a regional train down the western side of the Rhine gorge (Rheinschlucht) and see the famous castles, among them Lorelei. You could go as far as Koblenz and come back.
Or go hiking in the Taunus.