Frankfurt can occupy days of sightseeing if you know what you are looking for. This was one of Germany's' most historic cities. As a Free Imperial City, it was the selected location for the elections and coronations of the Holy Roman Emporer, site of the 1st and longest lasting Jewish Ghetto, home of the Berlin Airlift and the currency reform in 1948, and with one of the oldest churches in Germany you could keep yourself busy. Multiple Farmers Markets in the city center as well as the Klein Markt Halle, the beautiful Palmengarten, and lovely residential neighborhoods all built in the late 1800's. Check out Bornheim, Bockenheim, Westend, North end and of course the applewine pub district of Sachsenhausen.
One of the neighborhoods is Höchst and it is part of the Half-Timbered Route that runs through Germany. Here you will find a schloss with a dry moat, the Justinus Church (850AD) city walls, quaint cobblestone lanes with half-timbered houses, all located along the Main. (haven't managed to get Rick to visit here yet though, which is why it is not in the Germany book) Their market held 3 times a week is a delight.
When are your travel dates? There are so many festivals here and in the surrounding towns that I will be happy to make some suggestions.