L.,
Thanks for this question, I love to see mom's being consistent with their children.
We have a problem w/ our child not buckling up. He fools around in the back of the car while everyone's getting in, and when it's time to drive, he's sometimes not even in his seat yet. I started putting him in time out when we got to the destination, I have a time out spot at our regular park or if we're going home, but mostly I just need to find a corner or blank wall, you could do that at a rest spot before he gets to run the wiggles out.
This is just a tough age, and the most important thing is for you to never back down, and it sounds like you're understanding of his traveling limits.
My favorite thing for car trips is a potable DVD player! By the time they're 3 they'll watch Dora for hours. Also, you can use scrap paper and print out coloring pages of his favorite characters. I just do a google image search. Put it in a 3 ring binder and get a box of colors. Way cheep and custom built. You can get a new "book" for every trip. The thing that works the best is singing together. Also, children love their parents attention so play I spy with him.
Good luck, and happy traveling!