Helen:
A great deal depends on when you arrive in Frankfurt. There are some wonderful walking tours of the city, but they tend to start in the mid to late mornings.
With 12 hours to kill, I would look into getting out of the airport & explore. Just be aware that you will have baggage to deal with (carry on, at least). You will also have extra security checking.
But, YES, go explore! Have a few euros & have some fun!
Doug
