D.J.
I see you have lots of activities.........I try to take a bottle of bubbles, just a small one and blow bubbles in the grass at rest areas just a few minutes to let them chase the bubbles and work off a bit of steam. I go far away from the car traffic, over towards picnic tables are.
I pack an outfit a day per kid, a sweatshirt, and for your youngest I say a few spares. My older kids get one spare.
I also take a cup with straw and lid (even for my 11 yr. old) so if we get a large drink they are not passing it to one another but I can give them each their own drink that has a lid.
Lots of plastic trash bags, a roll of paper towels or lots of napkins. And hand wipes/baby wipes should be close by in the car.
Hotels....a nite light, if your room has a microwave pack a bag of microwave popcorn for a snack. Don't forget swimsuits, floaties, etc for the hotel pool.
I cut up some fruit and had it in plastic Cool Whip containers, etc. So we ate and tossed the sticky container at our next stop while driving. We took a few Go-Gurts in the cooler, I made sure they ate them while we still had a good supply of ice in the cooler and they were still cold. For my lil guy I have a container with sections. I made him a 1/2 sandwich, some cut up grapes and Goldfish so I could just give him that easily. I have a little tray I got at BabiesRUs he can hold on his lap to eat on, play or color.
If you are near Hershey, PA stop and do the tour of the Hershey Factory, and get the kids a milkshake. You don't have to spend a lot of time there, but they will love it!
Have a great trip!