Last time we went, I took "Grumpy Chips." They're circles of card stock with a sticker of Grumpy on each one. Each member of the family is assigned the same number of chips, and each one is worth 5 minutes of being grumpy.
Want a Mickey bar and can't have one? You can be okay with it, or you can use a Grumpy Chip and be as angry as you want for 5 minutes. The thing is, once you've used up your Grumpy Chips, you can't be grumpy anymore.
That begs the question: What happens if someone is grumpy even after running out of chips? I don't know. This worked really well for us last time, so I never got to find out what would happen.
But the idea is that it kind of makes you stop and decide if something is really worth throwing a fit over, and usually it isn't.
I realize it won't work for every kid/family, and I don't know if it'll work for us this time (kids are all two years older now) but it's worth a shot. This time I've coordinated the chips to match the color of each person's Magic Band, and I've allotted 10 chips per person. We're driving from Chicago so I hope it's enough.
Anyway, I also pack one of those super thin extra absorbent towels, moleskin pre-cut into various sizes to prevent blisters, those handheld mist things that you can pump by hand, ponchos, baby wipes, a travel thing of deodorant for the grown-ups, and sometimes a couple mini canisters of Play Doh for waits at restaurants.