Museum Viadrina is Frankfurt's main local-history museum, housed near the old centre. It is useful if you want the city to make more sense: the medieval trading town, the old university, the border, and the postwar rebuilding all sit behind what can otherwise feel like a slightly fragmented place.
The museum is a secondary stop after Marienkirche and the Kleist Museum, but it adds depth to a day in town. Check current opening hours before building your route around it.