I use a backpack at the parks that I got at the
disney store on clearence last year (best $5 I spent), obviously they won't be there anymore, but they may have an equivalent one soon. I bought the beach bag backpacks. They are mesh, so it makes it easy going into the park because security can just flip the bag around to see what is in it. It is smaller then a regular school size backpack, but I easily get DS's diapers, wipes, change of clothes, sunblock, snacks, my wallet, four pair of sunglasses, hand sanitizer, and extra pins for DD's pin trading in it. (The usual Mom supply list-

). Also, it has a padded strap. Not to mention Disney characters on them.
I've also seen people using clear backpacks because its easier to go through security with them. But I've read other dissers tales about how they feel like everyone can see what they have then. Personal comfort choice...
My only complaint about the backpacks (and this would be for any type) is that there isn't a good way to carry them in the strollers if you rent one. After trip #1 with them we had the idea of using the clamp clasps (I don't know if that is what they are called) and hooking the backpacks top handle through it and then clipping it onto the Disney stroller handle. Unfortunetly the clasp we bought for trip #2 was to small, so we are going to buy a bigger clasp for the next trip. (Whenever that may be...the sooner the better

).
Added info- The backpacks have held up really well, they look new still. I've used them for other trips too, like DDs school field trip, so I could carry my groups lunches and souviners and I didn;t feel bogged down with a cumbersome school size one.
Can you tell that I love these backpacks???
