Hey Clarissa,
I didn't spend a lot of time in Frankfurt because I wanted to see a lot of other places in Germany (Nuremberg is a really cool city), and mostly, I just wandered around Frankfurt, checking out the scenery and some architecture. But they have at least one big park, and some nice botanical gardens. I was going to go to the Goethe museum, but never ended up going. I was there about two years ago (and I was drinking quite a bit) so I don't recall much. I hope this helps you out a bit. Have a great trip!