We drive every year from NY so I am well prepared for things to do on the long ride! I have 2 kids, DD 8 and DS almost 3. Here's what I'm bringing for our trip in May:
IPods(one for kids, one for DH)
Gameboy advance SP(it also plays cartoons)
Video Now(preschool one for DS)
PSP(for DH and DD, they also have movies to watch on it)
Pixter(for both kids)
Color wonder markers, paper and coloring books(by crayola, they only write on the special color wonder paper)
Travel games such as Perfection, Connect Four, and Don't wake daddy(found these at Toys R Us)
Sticker book and stickers
Travel magna doodle
Activity pads, coloring books, colored pencils
Books for DD to read
Color forms travel pack
Now DD is usually good and other than having to stop often for the potty, these things keep her busy! The kids usually take about 2-3 naps on the way down. We usually only bring long movies to watch, like Harry Potter! It helps the time fly.