The pull-down art tables are great (they attach to the back of the seat)--you can put art supplies and snacks in them (I can't remember the name of the one we have right now, but you can e-mail me if you want me to look!). It's been a while since I've had a 1 1/2 year old, but if you are worried about art supplies w/the little one, there are those markers that only color on special paper. For the older one you can fill it with coloring or activity books, blank paper, crayons or colored pencils, maps, stickers (beach theme?) or whatever. Also, books on tape/CD can be great--something like the Winnie-the-Pooh books maybe? You could also get the actual book (or probably any Pooh book) for them to look at while they listen. The reading might even lull the little one to sleep. Maybe an etch-a-sketch? I've also brought a little bag full of new/silly dress-up stuff and a hand-help mirror. You could include masks, sunglasses, ears, scarves, a hairbrush, hats, jewelry--whatever you have around. We did the travel bingo w/my daughter when she was four (the squares have pictures of things to look for--e.g. a cow, a bus, a stoplight, a river)--though sometimes she may have "seen" something that wasn't there! I just try to keep everything by me to start and dole stuff out so that there is always something fresh for that long stretch of driving. Hope that helps. The little one will probably sleep more than the older one, so you'll need more stuff for your big guy/gal.
Have fun!