I see you are a Hilton Diamond. So maybe you should get a room for the night.

Check out the rates at the two Hilton Hotels in Mainz. The S-Bahn needs between 20 to 30 minutes from Frankfurt Airport to Mainz Main Station. Mainz has a nice old roman style cathedral, a nice old town, the Gutenberg Book printing museum which has a Gutenberg Bible on display, nice restaurants, a nice river promenade.

Alternative is Frankfurt. There is a Hilton which is almost downtown. Frankfurt has a lot of Museums, across the main is Sachsenhausen with a lot of traditional regional restaurants, nice shopping areas, the old town hall.