My must-have items are
1. Sharpie to mark drinks or snacks, and also for autographs
2. Meds- for in the park I have Neosporin, bandaids, bug bite cream, Advil for adult and children, Benadryl and one DD is on Claratin and Allegra everyday, so will add those. I have more stuff in the car, a full first-aid kit.
3. Extra clothes for everyone in the car (I got throw up on last time, and had to borrow DH's shirt to get home

)
4. Nail clippers and emory board
5. Mints, I keep a whole sack of them in the stroller.
6. 3 in1 sheets, I have a few left from the last trip, but probably should order another set if they aren't making them anymore!
I like the idea of the little Pringles cans! Great idea! I may try that, we take snacks in everyday, thus the sharpie to mark what belongs to whom.
We stay off-site at Bonnet Creek, and we stop along the way at rest stops to eat. I have a list of things I take for that: Roll of paper towels, bread knife, spoon and tomato knife in a large zip-lock, and stored inside the inner role of the paper towels. Extra ziplock baggies, salt/pepper shaker in the camping section, picnic table cloth, dishwasher tabs, aluminum foil or plastic wrap.