I can't eat popcorn anymore without getting sick, and the part that bums me out about that the most when I go to the parks is that I really liked the popcorn buckets. They are (or in 2009 were) pretty nice plastic tubs with lids that could be taken off and clipped to the handle. They were perfect for hooking onto a purse and you didn't have to eat all the popcorn at once if you weren't up for it.
Mickey ice cream bars have been mentioned, and they're my #1 must buy. My family has a tradition of taking a daily ice cream break at the parks, and the classic Mickey bar is my favorite. I also tend to buy one small plush toy per trip, maybe a couple pins. I probably would have bought a bunch of books last year, but I didn't have a smart phone then for price checking and I was leery of buying them and then finding out they were available on
Amazon for cheaper. Now that I've been to Trader Sam's once (maybe not relevant to OP's interests, with a two-year-old in tow) I'm tempted to get one of the tiki mugs each time, though that's fairly pricey (I love my Krakatoa mug and use it fairly frequently - right now it's a pen holder for a party tomorrow night, but usually it's my water glass whenever I want to be reminded of a fun time).
I guess my overall advice is the same as doing any shopping for non-necessities -- just ask yourself what you're going to do with it when you get home, and don't buy it if you don't have a good answer. "Put it on display" is a perfectly good answer if it's something you'll enjoy looking at and you have a place to put it, but I try to limit the things I buy purely for decoration.