Last year, we carried a Baggellini Messenger Bag that I ordered from ebags.com. (I ordered it after seeing several other people on this site recommend it.)
W/ 2 young kids, I knew we would need something bigger than a waistpack, but I didn't want the traditional backpack. The messenger bag was just right & had several pockets & little slots. In it, I carried the following:
change of clothes for DD & DS
Passporter w/ small notepad & pen
2 autograph books & 2 pens
sunscreen
lip balm
compact & lipstick (for me!)
small container of mixed pills for me & DH (Tylenol, Advil, Benodryl, etc.)
liquid Benodryl for DD & DS
small bottle of liquid Tylenol for DD & DS
bandaids
small bottle of first aid spray
Neosporin
small sewing kit
small bottle of antibacterial gel
travel pack of wipes
cell phone
my ID
"Key to the World" cards for me, DD, & DS (DH carried his in his wallet)
several rolls of film for the camera
DD's & DS's sunglasses
my water bottle
It was really cool because the bag had slots for all the bottles (sunscreen, first aid spray, liquid benodryl, liquid tylenol, antibacterial gel, lip balm) and zippered inside pockets for my ID & our "Key to the World" cards & the small container of pills, bandaids & Neosporin. The rolls of film went on one velcroed side pocket; my water bottle went on the other side mesh pocket. There was a specific place for the cell phone. The changes of clothes, Passporter, auto books, & wipes went in the large rear section. There was a small front section for my lipstick, compact, & kids' sunglasses. So even though we took a lot w/ us, it all fit neatly inside the bag which was compact enough that it was no problem taking it on rides. Really, it wasn't any bigger than a large purse. I wore it like a messenger bag, but, when DH carried it, he wore it slung over his shoulder like a slingbag/backpack. And, because of all the little compartments, when I went through the bag-check upon entering the parks, it was really easy & fast - I didn't even have to take it off - just unzipped all the sections. When we didn't want to carry it, it fit nicely on the back of the strollers as well.

Love my Baggellini messenger bag!