Take a S-train to Frankfurt and go to Hauptwache or Konstablerwache (should be a 20 minute ride). Lots of shops, restaurants (on the Freßgass) and the Römerberg and the cathedral for a bit of sightseeing. On top of the Zeilgalerie (a shopping mall at Hauptwache) is an IMAX if you like those kind of movies (and they are only 55 minutes long).