I am a messenger bag kinda girl. Backpacks make me CRAZY...I feel to constricted! SO...On trips with just DH and me, I carry an Overland Donner Bag. I love it b/c it has TWO diet coke bottle holders on either side of it! (that's actually what the pockets are for...diet coke bottles...seriously...

). It's got plenty of room for anything you might need. I found it to be very functional and roomy without being bulky or too big.
On trips with two kiddos along as well, I use a Diaper Dude messenger bag! (these are diaper bags marketed for men!

)It was such a blessing!! It has tons of pockets, both inside and out, a diet coke bottle holder, and a cell phone holder on the strap. I didn't find it TOO big...but it was plenty roomy and held four ponchos, hoodies for both kids, a first aid kit, snacks, my passporter, Pal Mickey fit nicely when we went on rides. It was a great bag. I don't do Disney without it!!!
Mostly I loved this bag b/c I could rummage through it without having to stop, unstrap it, and then reload. I could walk along and hand things to the kids if need be. It was very user friendly and it came in a really cool camo print that made ME very visible to the kids and my husband!!
Diaper Dudes come in TONS of colors. The best price on the web I've found is at travelingtikes.com. Free shipping too!!
good luck and have fun!!!