Frankfurt is honestly not a city that is anywhere close to the top of of German cities to visit, but the downtown is worth a few hours, and if you have more time, I'm assuming some of the museums are worth seeing. In my case, I had a bit less than 3H, so I just went for the easy sights:

I went to the financial district to visit the google office that was there. Nice views from there:

the cathedral had a small museum attached to it

I then went in the Römer Square area to see Old St Nicholas and frankfurt cathedral (that were severely impacted by the allied bombings during WWII, while the rest of the city was mostly leveled):

Römer Square was clearly built for tourists, and it is picturesque:

I then went to the bridge that has a nice view:

some electric autonomous bus prototype

of course, put locks on the bridge

Then, back to town to burn a bit more time:

Then it was time to go back to the airport for the terrible security line and treatment that was way worse than I was ready for, and onto my next flight to Prague. Frankfurt is of course worth more than 3H if you have the time to see museums, but it felt like it was more than enough for a visit: