Yes, fanny packs (or "waist packs" as Disney sometimes calls them) are generally allowed on almost every ride at Walt Disney World and Disneyland. Unlike some other theme parks like Universal Studios, which require all loose items to be placed in lockers for high-intensity coasters, Disney’s ride systems are designed to accommodate small bags. On most attractions, you can simply keep the fanny pack buckled around your waist; the lap bars and seatbelts are designed to fit securely over or around them. On more turbulent rides like Space Mountain or Big Thunder Mountain, the pack stays snugly against your body, which is actually safer than a backpack that might slide around on the floor. The only time you might be asked to adjust it is if the pack interferes with the proper locking of a safety restraint, in which case you can usually just slide it to your side. This makes fanny packs the "gold standard" for Disney gear, as they keep your phone, fuel rods, and wallet accessible without the hassle of the "bag drop" or locker routines found elsewhere.