I need to fly from London to Frankfurt on a Friday in mid August, to drop something off.

Haven't booked the return flight as yet

I was thinking of hanging around in that part of Germany for a couple of days and return to London on the Monday....

Is there much to do in Frankfurt?
Enough for 3 days?

Or would I be better jumping on a train and spending a few days in Munich, Salzburg etc and returning to London from there?

Just can't seem to find much of interest in Frankfurt on Google etc