We have been on a few long car trips with DS - the last one at about 22 mos old. We had 2 days in a row of being trapped in the car seat and DS did amazzingly well. We just had a goody bag of fun toys and books, lots of snacks and water, and we stopped when we could to let him out to run around rest areas (except that it rained all day both day - ugh!)
Toys he especially liked included a new one I picked up at WalMart (oh, I know, evil, but he LOVED the toy) - a little Winnie the Pooh noise maker type toy that plays different sounds and songs when you press the buttons. I can't believe I got it for him, but it was about $5 and, seriously, kept him happy for about an hour at a time. Other toys included little cars, little animals, hand puppets, lots of singing, a few favorite books, and a few small puzzles that he could take apart (and then droped the pieces on the floor so I couldn't get them until we stopped again). Oh, also markers and paper, which were fun but messy.
Safe travels!
