As the OP is not asking about any LH/M&M lounge facilities but rather raises the general question on what to do with 14 hours to kill at FRA I will move this thread over to the Germany forum.

Have a nice day

totti
Moderator M&M forum

If it were me, I'd rent a room at the Sheraton and take a nice nap. If you have the energy, I'd take the train to Heidelberg and spend the day there.

See below: the OP can get a room in the Kempinski for free...

I would follow SebFra's advice. And since it's free, you can always change your mind after arrival and go to Mainz (or wherever) anyway. (For me the weather on that day would be an important factor in the decision...)

Mr. Ericka and I are in a similar situation....actually 16 hours in FRA. Right now, we're planning to take the room on arrival to catch up on some sleep (body clock will make this necessary) and then rent a car and drive to Marksburg Castle for the afternoon. The drive will be about 1 hr.15 each way.

The free Kempinski hotel for LH first class passengers at FRA -- which should be booked well before getting there -- also has a free shuttle bus from the hotel to the downtown area. However, often it is already booked or not running frequently enough to be of use. And the shuttle bus to the hotel from the airport isn't always that quick to pick up as it is a bit of a drive and then some between the airport and the hotel. And the shuttle bus from the hotel to the airport isn't all that frequent always during the day despite its being used by LH as an "airport" hotel -- although that may just be a way to try to convince day-room guests to check out on a schedule that the hotel itself wants for its own advantage.