I'd suggest a "travel-sized" Etch-a-Sketch for your three-year old. (maybe your seven year old, too?) You can get them at Target.
We take a two-hour trip to Grandma's house frequently and we've got a backpack full of trinkets that our three-year old can dig through; Hot Wheels cars (they can amuse themselves just looking at them, running them up the sides of the car seat, etc), "guys" (action figures--preferably "posable" ones), that sort of thing. Really, anything that will amuse him in a restaurant will work in a car, too; picture books, etc.
Do make sure you stop every couple of hours so that they can get out and RUN. Major crabbies if they have to be penned in too long! If you're lucky he'll nap for part of the way and you can get more miles under your belt.
Once you reach your destination, be prepared to dance to his tune for a little bit and cut him some slack on behavior; he's just done what You wanted for the last few hours!
