I sweat a lot and like to have my arms free, so I totally appreciate your request. Stay away from backpacks... you will sweat even more than you would on a big hike in the sunshine.
I have a messenger bag that I LOVE! The thin strap has a small pad over my shoulder, the strap lays across my body, and the bag rests just behind my hip. There's another strap for around my waist if I need the bag to stay in place more than usual, like when I'm biking. The top of the bag folds over to clasp at the bottom of the bag, so there's no zipper to mess with when you are trying to get into the bag. This can be a drawback, though, if you worry about things falling out. My bag has two small zipper spots for cellphone and wallet or other small items. I used my messenger bag as a diaper bag as well. It is soft-sided and machine washable, too. I can't say enough good things about it.
I checked for the Novara brand and style that I purchased a few years ago, but I can't find the exact one. You can check out Timbuk2, NorthFace and other like brands for simlar styles. Your best bet is to go to REI or other outdoor store to try on bags and see what fits you.
Happy hiking!