A weekend in Frankfurt
Other than the museums, there isn't a TON to do in Frankfurt in comparison to cities such as Berlin & Munich. The river walk is gorgeous, and close to many museums. Other than this, walking around the old town, going up to the observation deck of the Main Tower and the shopping district could be done in a day. Gorgeous city, but not quite as 'Touristy' as the other cities that come to mind when you think of Germany.
Insider Tip:Try the Apfelwein (Appelwine). It's a local drink, and is similar to cider. Very good! Also, take a day trip to Heidelberg if you can (~35 Euros for the round trip). It has a classic feel, and is a one hour train ride each way from the Frankfurt Hbf (main station). Sachsenhausen is a great bar district, and fun to walk around at night.