Susan,
Thanks for asking your question to planDisney!
On most rides you can either place the backpack on the ride car floor, behind and between your legs, and you will be just fine. I did this all last week with a string backpack and had no problems. I even rode on
Guardians of the Galaxy: Cosmic Rewind with the bag, and no issues! With some of the more mobile attractions there often can be a small storage bag in front of your knees as well. But it won't hold a particularly full backpack, as you can imagine.
I will caution you, however, that there are a few roller-coaster level attractions where space can be tight. It is up to your discretion, but certainly if it is a particularly large backpack you may want to confer with the Cast Members before you proceed.
BONUS TIPIf you are feeling concerned about your backpack, there are
lockers available at all of the Disney theme parks. They are at the front of each park after passing through the turnstiles, and there are a few locker sizes available. They are keyless and very affordable for the day.
Disney's Hollywood Studios and
Disney's Animal Kingdom Theme Park have lockers that are as large as 15.5" x 13" x 17" (39.37 cm x 33.02 cm x 43.18 cm). The
Magic Kingdom Park and
EPCOT even have a few jumbo sized lockers available that are up to 17" x 22" x 26" (43 cm x 55 cm x 66 cm).
Please feel free to
write me back directly if you have any follow-up questions.
Have a Magical Vacation!
Doug