Römerberg (Römer square) or Frankfurt's "Old Town"
Overview: Frankfurt’s Old Town, Römerberg is the first, and probably last (to leave with a great impression), place you should visit in Frankfurt (aside from trade shows, that is!). It's beautiful and offers, in a very concentrated area, a lot of reconstructed cultural and historic sites and activities. Old Town is a reconstruction in the Altsadt area and there is no shortage of restaurants and bars.
Cost: Free
Accessibility: The square itself is accessible. The Museum of Modern Art and the Dom are as well. A lot of the restaurants and bars in the area are accessible, mainly due to the outside seating.
Getting There: Metro, walk / wheel