Where to stay close to FRA

Why don't you go straight to the airport, turn the car in, then take the S-Bahn into Frankfurt and spend the evening in a hotel there. You can take in the city for the evening and have dinner. In the morning, take the S-Bahn back to the airport and check in for the flight home.

We stayed in a town called Walldorf, just outside of the airport. It was a Best Western or some chain like that.

Since the airport is just a handful of minutes from the city centre by S-Bahn I was about say what Sam said but he beat me to it.

Chuck the car and spend the night in a city hotel rather than one at or next to the airport. One less chore to do tomorrow.

Do come into the city for the Luminale that is going on tonight. The Alte Oper and the Römer are stunning as is the ECB.
https://luminale-frankfurt.de/en/startpage/
Unfortunately because of the big trade fair being held too, hotels in this whole area may be quite expensive. Check on Frankfurt.de for hotels as they usually have some blocks of rooms set aside that aren't as expensive.

We have stayed at Hilton Garden Inn at Frankfurt Airport -- by our last night, we're ready to trade European charm for morning simplicity and just walk from the hotel to the gate.

The last time we flew out of FRA, we stayed on the Main near Würzburg and came into Frankfurt on the RE, but that was for an afternoon flight. Three times when I have had a morning flight out of FRA, I have stayed in Mainz, in a hotel on Schottenstr., right across the plaza from the Mainz Hbf. It's a little longer trip (27 min) by S-Bahn than from Frankfurt, but it's easy to get to the station from the hotel, and I think the prices are good for being so close.

The last time I stayed here, at Königshof, and was happy with my stay.