Oh gee, almost forgot! Unless you specifically request and reserve, the rental car will be a STICK shift. For fun, you can rent one of those smart cars for two - easier to park! Brush up on the driving rules, especially for the autobahn...still no speed limit outside the cities!!!! And the rental company will charge you up the kester for speeding tickets - automatic cameras everywhere. Make sure your credit card covers the car rental insurance, it's practically guaranteed a nick or two. Or like DH, sent in an ice storm...oops.
AND public bathrooms are rare and far between, so timing is everything.