Another vote for a messenger bag!
After seeing it being recommended on this site last year, I ordered a Baggellini Messenger Bag from ebags.com for our trip last May.
It's a bigger (& cuter) than a fanny pack & smaller (& more comfortable) than a backpack - at least to me, anyway.
Everything we needed fit, and it had all kinds of separate little compartments so all my "stuff" didn't jumble together in the bottom of the bag. Plus, I never had to take it off - unless I wanted to. When I went through the bag-check line, I just unzipped the pockets. And it wasn't too girly for DH to carry it either.
W/ 2 young kids, I couldn't get away w/ going into the parks w/ just our tickets & a water bottle, but everything I needed fit nicely in my messenger bag. Plus, I'm not a "light" packer - if I could, I'd pack the kitchen sink - but, again, everything I wanted fit in the bag - Passporter, change of clothes for the kids, autograph books, sunscreen, emergeny meds, bandaids, antibacterial gel, wipes, first aid spray, lip balm, camera film, sunglasses, lipstick, compact, water bottle, etc...
I loved my Baggellini Messenger Bag & plan to use it again this year!