You could just use distance sensors to walls but it depends on what you want to do with this project...
Is it for travelling long distances? - Use GPS
Is it for in an enclosed room? - Use distance sensors
Is it for on a flat surface? - Use better modified mouse
Is it for a small flat surface? - Use reference points
Is it a tracked or 4 wheel device which does not have moving front axles? - Use hall effect senors on axles - the slip is minimal if it is not accelerating too fast or on soft ground...
Regards,
Mowcius