At least to some people it's the most beautiful campus in Europe ;) Even if you're not a student there, you can access the area and take a nice walk. The architecture is impressive but comes from Nazi times. They've build new buildings on the campus in the style of the old buildings.
View Gallery
View Gallery
View Gallery
It's really beautiful. Also, local artists are sometimes allowed to place their artwork (like sculptures) either in the "frontyard" or in the back of the main building of the campus. If you are tired from walking and being surrounded by all the young and enthusiastic students you can just have a coffee at one of the many cafés.
You can also have a guided tour: https://www.frankfurter-stadtevents.de/themen/1110/20010050/