Frankfurt is boring
Not that much to see in town, and what there is to see isn't that impressive. But the Modern Kunst museum is a really cool building, whether or not you're interested in the art there. And everything to see is within a 5 - 10 minute bike ride of everything else, so you can see it all in a day. And because I'm from Seattle I love that they have a Hammering Man sculpture here. And it's more international than most European cities.
Insider Tip:Their bike-share system is easy to take advantage of. Not just use, but take advantage of.