We don't have a car in the city for lots of reasons (however anytime it's raining we can't remember of any of them), a big one being that parking here is bananas. The few times we've borrowed a car we've come close to hysteria looking for parking, and then having the piece of mind to remember where we parked the next morning is a whole other issue. But here's a helpful way to avoid the insanity.
Instructables has a super-easy project for creating a magnetic arrow key hook. When placed on a map (that's on a magnetic surface, of course), it points you to your parked car... that is, assuming you have the clarity to recall where you parked by the time you get home. We wish it was a bit more attractive, but there are loads of ways to customize it to fit your aesthetic. We might try using a simple magnet (perhaps like these) and leave off the whole hook thing. You could also forgo the Sculpey and simply hot glue a magnet to a little toy car or something - plenty of options.