I found this bag on clearance at REI. I think the color was from last season or something. I really liked it because it was made to carry two full sized water bottles. Anyway, I have since seen it also discussed on this board. Here's a link with a picture and a detailed picture that shows the organization of the front section:
http://www.tworoadsfitness.com/ovdonbag.html
The two side pockets are great for holding the water bottles. I use Nalgene refillable, unbreakable bottles in a slim, tall style. I find the loops on the tops of the bottles make them really easy to get in and out of the side pockets, too.
http://www.rei.com/product/729143
I find the front pocket to be great for organizing and getting at things quickly. Where the "Black" notebook is shown in the picture is where I keep our family autograph book and pens. I also really appreciate the zipper on the bottom part for putting our keys, photopass, FP, etc. There is also a no zipper outside pocket for putting the park maps, etc, and other things you want to access easily.
I can easily fit ponchos, snacks, wipes, sunscreen, camera, etc in the main compartment.
The bag isn't too big. It has a nice long strap for wearing over the body. It stows well on all the rides. The material is very sturdy, so I don't feel bad leaving it on the floor of the rides as well. I did find mine for a great deal, so shop around. The link I gave doesn't have the best price, just access to the best picture.
Happy shopping!