Hi Julie and welcome to planDisney!
A park bag is essential when visiting the Walt Disney World theme parks, especially if you have little ones traveling with you. Since my kids are getting older, I've gotten pretty good about carrying the minimum necessary, but we always have a backpack with us. I dream of one day being bag-free!
I am happy to tell you that there is only one attraction at Walt Disney World that does not allow backpacks and that is the brand new
TRON Lightcycle / Run at
Magic Kingdom Park. Complimentary lockers are available for all of your belongings that you do not want to or can't bring on the attraction with you. The locker process is pretty cool, too! They are located inside of the attraction queue just before boarding and then you pick up your belongings right after your ride ends. Backpacks are allowed on all other attractions and can be stowed at your feet while you are riding.
There are
lockers for rent if you do not want to carry your backpack around with you all day. There is at least one set at each theme park and they are usually located towards the front of the parks. These lockers have in and out privileges all day long for just one fee. They are very useful if you purchase souvenirs throughout the day and don't want to carry them around.
Please
stop back by and visit if you have any additional vacation planning questions.
Have a magical day!
Leslie