8 hour layover in Frankfurt

Frankfurt Airport is fairly efficient and you can be out and on your way into the city in about an hour. Trains to the city run every 15 min, and it only takes 15 min. to get to the city center - the Hauptwache. Plenty of things to see there, but it depends on your interests, when you are going to be there, time of day, etc.
Showers are available in both terminals, I believe for 6€.
I would certainly go into the city rather than nap.

The Frankfurt airport isn't bad now. I first flew there in 1996 and didn't like it. The airport was a utilitarian relic of the Cold War era and crazy with crowds. Nineteen years go by, and I flew into Frankfurt in 2015 on my way to Warsaw. What a improvement! It felt relatively comfortable. They must have renovated it during the interim.

I noticed both times that the food is much better than in most airports in Europe, American, the Middle East or Asia.

Frankfurt doesn't have quite the charm of other German cities, but some ideas for a quick visit are Kleinmarkthalle, a bustling market hall with food stalls, restaurants, and shops; Goethe House, the birthplace of the author Johann Goethe, restored to reflect its 18th-century setting; and Romerberg Square, the historic city center.

The airport has "nap cabs" and showers. I walked right past them on my recent arrival. Here is the airport's page on services:
https://www.frankfurt-airport.com/en/services/facilities-and-services.html

And there are four hotels at the airport. The Squaire has loads of restaurants, a Starbucks and a large supermarket.

P.S. Hello, fellow 'Hamster!

At one time Frankfurt on Foot tours offered a "layover" tour. I don't know if this is still an option, but our guide was outstanding (Ms. Jo, who posts on this Forum). Super easy to take the train from the airport to the city. I found Frankfurt to be very interesting!