Thats were im flying in to
We also flew into and out of Frankfurt. Whatever transportation you are using, take it out of Frankfurt and head to Bacharach (along the Rhine), or Cochem (along the Mosel). Both small towns are quaint and fun to visit. We enjoy staying in Bacharach our first night and eating at restaurants with authentic German food and wine. My husband and I lived in the Frankfurt/Mainz area for 3 years. The only reason to be there is to fly in or out of.
You can take a boat trip on the Main, which is very pleasant and interesting. Frankfurt is one of the bombed-out cities which chose to rebuild in the style of the day (1950's--not the most attractive era) rather than to restore its original style. If you find "Bankfurt/Mainhattan" dull, there are lots of nice places to visit in the area via relatively short train ride. Try googling "Hesse" or "Taunus Mountains."
Great nightlife, plenty of sights, hundreds of museums, dozens of great castles nearby... what is your question?