Driving is easy. Trying to avoid crazy out-of-country fees by switching cars in each country is one option, but not ideal. If it were me, I would plan my driving itinerary to form something like a loop in order to it off in the same country I picked up in. This may involve some backtracking that you might be philosophically against for obvious reasons. But remember the scale difference involved here. The size of these countries are like the size of western US states. It's not really that much of a drive, for example, to get from northern Italy to southern Germany. I could easily envision a pleasant driving route that involved a pick-up in Frankfurt, a path through much of France, Tuscany, Venice (park & boat) and then drive back to Munich to it off. You could even make the lovely small city of Salzburg, Austria your end point and off the car 2 miles away in the German suburb of Freilassing. Trains from Salzburg or Munich to any other nearby city with better airport connections are quick, quiet, comfortable, and can be had very cheaply if purchased 90 days in advance online from Bahn.