There are a couple of things that we did not expect when driving (aside from what has already been shared). I lovingly call them lessons learned from the road.
One of my friends told me that he had a hard time renting an automatic transmission in Europe. It didn't matter to us, as we both can drive a manual. But I would put that in your request when reserving a car.
Our manual car also turned off when at a stop light (as a green thing/fuel saver) and then would automatically turn back on when we engaged either clutch or the accellerator (I cant remember now). It was strange, but cool.
Also it is illegal to run out of gas on the autobahn in Germany. (So make sure you fuel up)
The gas stations we came across (this might not be for everyone), you pay after you fill up. I went in and asked the first time, and they looked at me like I had three heads.
We originally didn't request a sat nav/gps (Sorry too much British Top Gear). But one came with the car. We were so thankful it did. It helped us find the out of the way places/hotels/family reunion spots/etc.
Familarize yourself with the international road signs. The one that I thought was a do not enter, was actually a no parking. Looking back on our first day driving, it is funny, now. But it was a bit stressful then.
I don't know how many of you will be in the car, but if it is two, then a small compact car will be the best. Roads are a lot narrower than they are in the States, There were several times we were glad we had a smaller car than what we drive here. There was this one time in Stuttgart where there was a garbarge truck and parked cars on either side of us leaving not a ton of space. And another time driving up a steep narrow hill to reach our hotel/castle.
For parking garages (at least in Germany) you pay the ticket at a kiosk before getting in your car to leave. You have a small window to do so. One time we waited too long to leave (we were looking at directions and for a sweater), and had to go put more money on the ticket, and ended up getting honked at by some very angry Germans. I wanted to die of embarrassment.
Make sure to stay in the right hand lane when driving then the left when passing on the Autobahn, otherwise you will have some very unhappy people with you. There were times that we looked over to pass the car in front of us looked back to put the turn signal on (a MUST) and a porsche/bugatti/ferrari shoot past us going 100 mph.
We did have fun driving and we were there for a family reunion which took us to some out of the way places where a car was a must. I look back and am glad we drove.