need ideas for 12 hour lay over in Frankfurt

Safe possibly, but... Are you staying near Frankfurt's main station? It will likely require around an hour's time to get your bags (if any,) get through passport control, get to the train station, get a train into Frankfurt, and walk to your hotel. So maybe you'll make it to the hotel by 10:45. That's if your plane is on time. Then the next morning you'll need to be at the airport early as well, so you'll need to leave Frankfurt maybe 2.5 hours prior to flight time, I imagine, so that you get through security and to the gate by loading time. So you'll need to be finished with breakfast and packed up and at the station by around 7:30. That leaves you a little less than 9 hours for checking in, sleeping, showering, packing up, breakfast, checking out, and getting back to the train platforms. I doubt you have much time for anything else at all.

If you stay at an airport hotel, you'd obviously have more time for just stretching your legs on a walk around the airport. It's a huge place where you could no doubt get your hear rate up.

Since you imply that you will have the energy go some place focused on your preferred form of entertainment and pay attention like you do when you go out in a big city at a similar time.

With that late arrival and early departure, just book a room at the Hilton Garden Inn or the Sheraton. You have no time to go sightseeing in Frankfurt, unfortunately.

Thanks for the advice.