First, bad design: I find traditional school-type backpacks very un-useable. They are really just shaped to carry books, will never sit on the base without falling over, and the zip down design causes things to fall out everywhere when unzipped. I'm not a big fan of the scratchy material used.
Better design: I won a Flash-18 lightweight daypack from REI at a trail race last summer. It has an interior pocket that is supposed to accommodate a Camelbak bladder for use when running/hiking/etc. I tried that feature and it simply sloshed all the time; the chafing from the pack would have been horrible if I had run in it. But I liked the lightweight design, breathable mesh straps, built-in emergency whistle, narrow profile, top-loading, and quick-pull closure. It also has a mesh interior pocket that zips closed. I keep a camp spork and chapstick there, and sometimes my phone and wallet. The fabric is very lightweight parachute-type tech material. Since I wasn't going to run in it, it sat around a while. Then in the past few months it gets daily use...first and foremost as a simple way to carry my change of clothes to the bathhouse when I shower (I'm a liveaboard) and second (and very handy) as my go-to "dinghy purse." Getting in and out of the dinghy is a two-handed task, so I use the pack to carry snacks and drinks and whatever if I am dinghy-ing over to visit friends at another marina. I expect when cruising it will serve the same purpose...handy for carrying my wallet, a light jacket, and for bringing smaller items back to the boat if I do shopping and exploring ashore.
Wish list improvements:
- I wish it were waterproof (I guess I'll spray it with 303).
- I wish it had a handy place to attach a handheld VHF radio and handheld GPS in case of emergency (to use as a ditch bag, e.g., or to have handy and attached to me in foul weather).
- I wish it had a wider selection of colors (it's a yucky guacamole color, I would've loved graphite or purple).
- I wish it had a way to expand for when I need it to have a wider capacity; perhaps zippered or snap-out gusseting. (But in general I like it's narrower design, which is the perfect size 85% of the time.)
- I wish it had built-in or attachable dry bags for wallet and phone.
I hope that helps your research! I'll be happy to test out your prototype.