My folks live 3 hours away by car but it takes us 4 hours. We always leave at nap time, on the way home last trip he slept for more than 2 hours! We always stop at a park too, not only for nappy change but so he can stretch his legs, we sometimes stop at the shops or something instead, just to take his mind off things.

We have a DVD player too and it really is a life saver, we use it for the end of the trip though, we he gets cranky. He has his leapfrog with him with he plays with for ages or the passenger will read him a book or ten.

Plus take lots of water and food and a wet washcloth in a plastic bag...