Hi Farrah!
I almost always have a medium-sized backpack with me at Walt Disney World on every trip. I use mine to hold a camera, snacks, and sometimes a poncho or sweat shirt. There are no ride restrictions with regards to backpacks. You can bring a backpack, purse, diaper bag, bags full of souvenirs, etc on every attraction in all of the theme parks.. What you do with your bag depends on the actual ride. For example:
- For most of the attractions, you can just put your backpack on the seat next to you or on the floor of the ride vehicle
- Some attractions vehicles have a dedicated bin in front of you or under your seat that you can put a medium backpack into
- For some of the more thrilling attractions, you can put your backpack on the floor with your foot through one of the straps
I have never been concerned about my backpack or the contents except on Kali River Rapids in Disney's Animal Kingdom. On that attraction, almost every guest gets wet and some guests get really soaked. However, in the center of the ride vehicle there is a place to put your backpack and it is covered (though not necessarily water proof).
The only negative of bringing a backpack is other members of your party will ask you to carry their stuff! I've never figured out a solution to that problem! Enjoy planning your trip to Walt Disney World, and write back if you have any more questions!