Suggestions for where to relax at FRA airport

If you have a Chase Sapphire card, you can access the Air Canada Lounge at Frankfurt Airport.

Frankfurt Airport Leisure Zone

Do you have Priority Pass?

Lux lounge in Terminal 1 or
Priority Lounge in Terminal 3

Do you have American Express Platinum?

There are several Lufthansa Business Lounges you can access. If traveling business class you can access Lufthansa Senator Lounges.

Most lounges have a 3 hour limit.

Look at hotel day rooms, looks like they run around $100.
https://www.dayuse.com/s/deutschland/hessen/darmstadt/frankfurt-am-main/frankfurt-airport-fra

With that long of a layover, pop into town. Tons of stuff to see, there might be a fest downtown, lots of good restaurants, maybe a museum. You would have a good 3-4 hours sightseeing time and the trains only take 15 min. to get to the city center - the Hauptwache. S-8 or S-9, and there is one every 15 min. No reason to sit at the airport for 8 hours when a sunny day in Frankfurt is such a nice option.

Consider going to The Squaire. Directly attached to Terminal One. Restaurants, a few shops, gym. I also think there is a spa attached to the Sheraton/Marriott.

Thanks to all the great ideas.
I noticed that The My Cloud Transit Hotel is inside security. How good an option?

Expensive pods. I would not want to sleep in them.
Security is not that big of a problem in Frankfurt and doesn't take long. No real reason to stay airside or even to stay at the airport for 8 hours.
If you want to sleep, and shower, get a day room at Hilton Garden Inn.

Thanks, Ms. Jo,
The Hilton Garden Inn, is in The Square, right outside the exit. If going through security is not bad, that is a great suggestion.

There are restaurants at the Squaire too, as well as a REWE grocery store and Rossmans. Paulaner has Bavarian food and it is also in the Squaire. Opposite end from the Hilton.