I wear a fanny pack for that exact reason at Universal. You can go on any ride that requires lockers with a fanny pack including Men in Black, Mummy and Harry Potter with fanny packs. I have been on Harry Potter many times with a fanny pack. No problems. Be careful that they are closed tightly because you are on your back many times with your hips slightly higher than your head. Not to say that you're close to upside down in any way but there is a slight tilt at times that I automatically clutch my fanny pack!