10 hour layover in Frankfurt

I've only been to Frankfurt once, over 10 years ago, so I'll let others answer your question. But I do know they are going to need the following specifics to give you the best advice:

Exactly what time are you arriving, from where, and in which terminal?
Exactly when is your next flight, to where, and from what terminal?

If you don't know the terminals, specify the airlines.

All of these particulars are important. For instance, if you're arriving when museums are closed, people need to know that before recommending them.

If you haven't already booked this (to Slovenia via your other post), I might reconsider and fly into Venice instead if you can arrive earlier in the day. You can shuttle to Ljubljana from Venice in about three hours.

Frankfurt on Foot does layover tours.

http://www.frankfurtonfoot.com/