I bet you are getting so excited, I wish you a great trip!
To answer your questions…
I would try a hipster or a small backpack, not a fan of the fanny pack!
Personnaly I would not trust anyone else to watch my stuff unless they were in my traveling party. You can never be too careful! Like someone else said, some of the rides have spaces to hold your stuff while you ride, like Kali Rapids, but in order to keep the things you have safe and dry I would keep cell phones and the such in zip lock bags inside your choice of item to carry them! Zip Lock bags are a saving grace at Disney!
I would also just leave a credit card on file at your hotel, and everything will be charged to it. It helps us keep track of our spendings and then we get the one bill at the end. When I get home (or even before, if I remember) I just take the cash and pay the card. (This way I begin accumulating rewards on my Disney Visa for next trip! ). I of course always have our debit card on us incase we need cash, and we always travel with some cash as well!
I tend to bring a mini flashlight when we go, I find it useful at night when we need to find something in the backpack, stroller, my purse, ect… It just hangs on the backpack, so light it adds no weight at all!
GUM!!! If you are a gum chewer, you will not find gum sold at all on Disney property, so you must bring it with you!
A poncho and/or umbrella (you can buy both in Disney, but why would you if you could buy them at your local $ store for as little as a $! We have never taken a Disney trip that we haven’t experienced rain. In florida it usually doesn’t ruin the day, but it tends to shower here and there, and it seems to help with the cool off! (If you choose a small backpack to carry and are wearing sneakers, pack an extra pair of socks!)
I could go on and on, but I will stop here! You get the picture!!! Have a blast!