I went to Germany when I was in high school, went to Paris for a couple nights but mainly stayed in Germany. Definitely do the Porsche factory tour and Mercedes museum if you can, I went while the new museum was being built.
We stayed in a small town near Ramstein AFB with some people who used to work for my parents, he was a gov't contractor. As has been said, spend some time driving around without any real destination. Obviously driving the autobahn is a big plus as well, but you get used to it quickly.