When I went on my self directed trip to Europe a few years ago that was one of my biggest fears - forgetting where I parked my rental car in all of the places I was visiting. Think about it - if you lose your car what can you do? I don't speak the language, don't know where any thing is at - like the police station. So I carried a notebook and pen with me just so I could write down the address of the place where I parked my car every time ( it also works well to write the prices of items you are negotiating to buy at the antique store or flea market if you don't speak the language). Driving/ parking/ reading street signs was the worst part of the trip.