X.O.
One game my 4 yr old and I like to play is "I Spy." Kinda gets repetitve in the car (for me) but he has a blast.
Movies:
Cars (very appropriate for a drive)
pretty much anything by Disney/Pixar
Mary Poppins
Music: Do some kid-friendly sing-alongs, like Laurie Berkner (sp?)
Introduce them to "The Beatles"
Also have some relaxing music for naptime
Silly putty
Be sure to let them do lots of running at the rest stops