Last year, at the recommendation of several people on this site, I purchased a Baggellini Messenger Bag from ebags.com for our trip to WDW last May.
I loved it!
It was smaller (& more comfortable) than a backpack & bigger than a fannypack. It had several pockets & slots for all our stuff, & none of the things ended up in a jumble in the bottom of the bag. It also wasn't so girly that DH wouldn't carry it. When going through the bag-check line, I didn't even have to take it off - just unzipped the pockets. And I wore it on rides w/ no problems as well.
I'm not a "light" packer; if I could, I'd pack the kitchen sink. I was able to pack the following in the various pockets & slots of the messenger bag w/ no problem:
Passporter w/ small notebook & pen
2 autograph books w/ pens
Change of kids' clothes - granted, size 3 & size 5 shorts & shirts don't take up much room!
4 rain ponchos
Small bottles of Sunscreen, Bug Spray, First Aid Spray, & Antibacterial Gel
Bandaids
Neosporin
Small bottles of Liquid Benodryl & Liquid Tylenol for kids
Small Container w/ emergency meds for DH & me (Advil, Benodryl, Tylenol, etc.)
Small Sewing Kit & Scissors
Lip Balm
Lipstick
Compact
Kids' sunglasses
Antibacterial gel
Wipes
Camera Film
Cell phone
Mine & Kids' "Key to the World" Cards (DH carried his)
My ID
My Water Bottle (Kids' water bottles were clipped to their strollers)
Snacks for the day (fruit roll-ups, small packs of crackers, etc.)
(The only thing that would not have fit would have been sweatshirts or jackets if we had been there during a cooler time of year. Of course, if I had taken all the other stuff out, I probably could have fit a sweatshirt or two in there.)
I loved my messenger bag & plan to use it again this year!!