12 hour layover...
...get a room at the Sheraton FRA and possibly train to Mainz for the afternoon, or train into the city, use the Westin as a base, and explore a bit? Know this is more of a "what to do" question than a pure hotel one.
I know Frankfurt doesn't have the reputation of other more interesting Germany cities, but I am curious if it's worth ~6 hours of walking around (accounting for a nap at the start and transit back to FRA at the end).
Thanks much.