Just use the search function in this forum and in the M&M forum I can remember at least 4 threads in the past month where we discussed this issue.

mosburger, maggie6872 and number_6 still posted some interesting information and you'll find a lot more in the other threads. And as maggie6872 pointed out it would be also important to know the day (weekday and month) you are in FRA. For example mosburger's advice makes much more sense on weekdays at the spa in BAd Homburg is quite crowded on weekends IIRC.

Anyway there a lot of things to do and there is no need just to hang around at the airport doing nothing… Enjoy your layover.

Cheers
totti