We bring a neoprene lunch box with frozen drinks (water bottles and juice pouches), nuts and cheese, apple slices, celery sticks, and protein bars. If our hotel has free breakfast, we'll take donuts, bananas and yogurts to go. Just remember that no glass can come through security! There are so many blogs for best value snacks, and I've had great experiences with mobile order, but I feel like we usually get our snacky urges once we're already in a ride line. At Disney, counter service restaurants will give you a cup of ice for free, and you can fill it at a water fountain or I think they usually have spigots by the soda machines. In Florida we bring flavor drops!
We don't bother with ponchos in the spring/summer/fall at Disney World because it is too sweaty, but at Disneyland we'll have a packable windbreaker or puffer (32 Degrees makes a good one!) We also have bandaids (also available at first aide, but you don't always want to walk to get them), sunglasses, chafing gel, and Starbucks gift cards (if they're not loaded to your app)! I haven't been since I got my Loungefly, but I've read a tip to bring a plastic bag to put them in on Kali River Rapids.
If you are bringing a stroller, you're better equipped to bring your own souvenirs if you want to save on ears, bubble wands, light up necklaces, autograph books, etc. by ordering them online in advance. You can also bring cushions to sit on if you're planning to scout out spots for the parade. I've seen guests reporting differing experiences with collapsible stools, but I would say skip it, because you don't want to risk having to toss them or take them back to your car/hotel.