Is is cheaper to arrange a rental online before I leave the U.S. or when I arrive in Frankfurt? I'm not sure if I want a car for the entire 3 weeks but if I rent online I think I'll have to commit to the whole three weeks.
Roy:
It is better and cheaper to arrange from here.
I recommend gemut.com. An Oregon based company with great German car rental and other travel connections.
Also, if you are getting a car for 21 days you should consider the lease/buyback programs - very reasonable prices. Technically you are buying the car and the manufacturer buys it back (it is a 21 day minimum). All insurance, etc is included. Both Puegeot and Renault have such a program. I have done this twice and it worked splendidly both times.
Have fun.
Another strong recommendation for Gemut.com. Call them - it's a no-brainer.