Any other city than Frankfurt, for a start—but if you must, you must: The Römer is interesting, and there’s a decent Zoo. Also Wiesbaden is half an hour by train, very nice town. The cathedral at Speyer is phenomenal, also not far away. If you like towers the observation deck is nice.

There are some great bars that make their own apple wine. I’d check that out. I didn’t go into the zoo, but it looked awesome from the outside, always something to check out. If I were you i’d start walking by the opera house and check out the shops around there, and walk down by the river and walk down it, very beautiful and looks awesome on the river at night.