My DS (almost 3) loves his little magnetic dry erase board during car trips! (It was very inexpensive at Wal-Mart, and just the right size for a toddler lap.) I collect various flat magnets in a baggie, and have one or two markers for him, as well as tissues to clean it off. He spends hours arranging and rearranging the magnets and drawing on the board. We'll even play games with it: I'll draw a picture or letter and he'll try to name what it is, or he'll "draw" something for me to guess. (Obviously, DH is driving at that time!)
For stretch breaks, you can't beat bubbles! DS loves for us to blow bubbles for him to chase, or vice versa. We usually take a small "non-spillable" bubble tub and a bigger container to refill from. Bubble play gives everyone a chance to stretch and move, and get out some of the pent-up energy.
I've also had a lot of luck with taking a LeapPad. If you have one (or whatever LeapFrog system is appropriate for your kids) or know someone who does, bring it and a few extra books. It's entertaining _and_ educational, so I don't feel so bad about using it as a "traveling babysitter"!
I also couldn't live without our plug-in cooler, as someone else mentioned. It keeps drinks and sandwiches cold, and also keeps candy and such from melting if we leave it in the hot car during a mall stop or stretch break. You will probably want to turn it off if the car will be off for any amount of time (to save your car battery), but we've found that it holds its cool very well.
Sounds like we'll be there the same time as you! We leave Friday and arrive on Saturday evening. Maybe we'll see you there!
-Bonnie