Driving from Germany to Poland is like driving from one US state into the other. Keep in mind Poland has very different (and weird) speed limits vs. no speed limits at all on German freeways. In Poland really everybody slows down to 40 km/h the second the cross a city limit (it would be 50 km/h in Germany but people consider it more like a government recommendation rather than an actual speed limit). Gas is much cheaper in Poland than in Germany. Make sure you tell your German car rental provider upon pick up that you're taking the car into Poland as then they can't give you a German brand car for insurance reason. You'd then end up driving a Ford Focus, French car, Skoda or Asian brand. Görlitz on the Saxonian side of the border is particularly worthwhile...