I have a Donner bag and I was really happy with it as a WDW bag. The bottle holders are GREAT. Very handy. It's a pretty roomy bag overall and I had no problem taking it on rides and such.
Although my husband wasn't all that keen on holding it b/c he said it "looks like a pocketbook..." (which is New Jersey for "purse"!)
We are going again this summer but are taking the kids this time. I was actually pretty concerned that my Donner bag wouldn't hold everything I might need as the designated "pack mule"!

SO I found a FANTASTIC bag that I think will work! I just got it. It's a Diaper Dude Messenger bag!

It's marketed as a diaper bag for "hip dads". It's completely unisex and no one would know it was supposed to be a diaper bag. I LOVE IT! It comes in a green camo print, all black, brown, navy blue, a really loud plaid, a pink camo print (the diaper diva), I think gray and orange as well. It's comparable in price to the Donner bag and it's TONS of compartments, a "bottle" holder that fits a diet coke bottle cuz I checked FIRST THING! It's got three large pockets on the front, pockets on the back, a large inner compartment that comes with a little changing pad which you can take out of you don't need it, and a cell phone holder on the shoulder strap.
I got the green camo print bag. It's not too huge, but big enough to carry everything you might need. I think it's going to work out great. I'm going to use it as a carry on to the plane as well. I can use it for school too.
I got mine on @
www.travelingtikes.com . They have the lowest price. These bags are on Ebags.com for $61 and up. Traveling tikes has them for $49.99, no matter what color you choose. Their customer service department is fantastic and you get free shipping! I really researched this and I think I got the lowest price and the best deal on the web. The bag I got sold for $65.99 at every other site.
good luck!! If you go with the donner bag it comes in lots of fun colors too! Check around those b/c prices vary by website!