Thanks for the ideas! We've tried singing (works occasionally to entertain him) and for a long time I had only kids CDs playing. I can often calm him these days by offering to tell him a story ('about Owen and Mary the big bad wolf and the giant and the wild things,' lol). But none of these things seem to help with actually getting him into the car, even if they keep him somewhat entertained once he's there.
A lovey that's only for in the car might help. He just recently started napping again at school and his napping coincided with his bringing in a stuffed snake that he likes.
He is artsy, but they do tons of art at daycare, so I'm not sure it would be a draw. He adores puzzles, though; perhaps I can get him a lap desk and a set of car-only puzzles.
We have an ispy bag but he only likes it when I play with him.