Ziploc bags for our iPhones. You can still use the touchscreen through the bag, and it offers a little protection on water rides.
Jackery external battery charger. It tucks inside my backpack and allows us to charge our phones on the go inside the park. We only use them for the MDE app and photos, but those two things seem to drain the battery quickly!
Ponchos, not only for rain, but for wet rides. Beef jerky, trail mix and granola bars, to save money when you want a snack AND to prevent sugar letdown.