Be prepared for the high costs. If the driver is very friendly you can tip him with DM 5. If you got a car from a car rental agency, please inform yourself on the traffic regulations, because traffic can be fast and dangerous. Ask for the way, and make sure there is a parking lot at your destination. In the city, watch out for bicycles.
Gasoline is quite expensive, but the cars are economical.
It is advantageous to have cash money, because credit cards are only accepted in the bigger shops and restaurants. Check out the customary stickers at the entrance. You can carry cash with low risk, because mugging does not occur frequently. Beware of pickpockets at Marienplatz and Hauptbahnhof. Like everywhere in the world, telephone calls from the hotel are expensive.