Looking good Ken!

Other sensors...
IR - Probably not do much better than the sonic, but they can be setup to "see" light and dark colors. Line follower. Maybe see the white wall...

Hall-effect - Have the car find and park or something on a metal plate, magnetic.

Solar cell or photo diode - Light seeking. You mentioned this at the beginning.

RFID - Place RFID tags along a path and have the car follow. (never done that myself)