Their some essentials to have in your bag for the parks(or in my case my vest): Band aids, ponchos, pen, sunscreen, antibacterial wipes, tissues , cell phone, phone charger/battery pack, park maps, Magic Bands/Tickets, credit card, photo id, cash, gum, tylenol/motrin, and zip lock bags.
Some extras that are nice to have: wet wipes, lanyards (and pins if trading-I also carry spare backs), autograph pen and book, snacks, towels, and spare pair of socks. I alos carry benadryl, anti-naseous pills and Immodium (just in case, I use tablets).
If you are bringing a camera make sure ot have back up battery and memory card.
Remember Selfie Sticks are banned in Disney parks, as are chairs and large coolers.
I also carry a bottle of water at all times as you do NOT want to get dehydrated!!!