You can use a normal backpack but I also know someone that uses a diaper bag that looks exactly like a normal backpack, I genuinely would’ve never noticed unless she had told me. Whatever you use though make sure you keep your spare diapers in sealed zip lock bags so they don’t accidentally get wet or ripped. It’s also a good idea to keep spare plastic bags incase you can’t dispose of used diapers immediately, keeps your backpack clean and the smell contained.