Now if you want a more low-key back-pack, Jansport or Columbia brand backpacks are sturdy and full of neat little compartments.
If I have to have a bag at all, I MUCH prefer one compartment. I don't want to be the person holding up the line at bag check.
I used to bring a mini Jansport backpack, but as we walked, the zipper would slowly un-zip, and stuff fell out. I tried clipping the zipper shut, but that was a pain.
I've tried inexpensive drawstring bags. Bonus- you won't care if it gets lost. Downside- the string straps dig into your shoulders after a while. If you find one of the new hybrid ones with wider shoulder straps, those can work.
Our main strategy is to minimize what we carry. The second strategy we try to employ as much as possible is using just pockets. Guys' cargo shorts are great at WDW.
I've tried almost every bag over the years. Any bag = having to stop at gate check. Not a huge time loss, but I love the freedom of avoiding it! Summer = bottled water though. Big bags are tiring. Big bags+heat= sweaty back.
Backpacks have to come off your back for most rides, so that's my other reason for avoiding them. If I must carry a bag, I prefer a small crossbody bag. It never has to leave my side. Just drape over my shoulder and forget.
That said....we also own a small backpack cooler. Mostly we only use it outside the parks (beach trips), but it is nice if you want to keep food cool for a few hours. I found it at Target; the "Picnic Time PTX" looks like it. The straps are padded and it only has two pouches to unzip. It also has drink holders on the sides. If we bring it, we all take turns.