I find the REI Stuff sack to be too deep to use as a SLR toting day pack. I'm short, though, and that also applies to my arms. My SLR would fall to the bottom of the bag and pulling it out seemed like a hassle.
Check out the REI Ashbury Daypack (ignore the pink pulls if you're not into them)
http://www.rei.com/product/846035/re...daypack-womens
A male family member purchased their last two versions (lost one, replaced the lost one) which had a different name but a similar form factor. It will easily swallow up an SLR like a Nikon d7000 and a tablet/eReader. Even though it is a women's bag, it isn't too feminine while zipped if you replace the pulls.
It is small enough to pass as a purse size if carried by your girlfriend which is handy for museums that don't allow school sized backpacks.
The Patagonia Lightweight Travel Tote also converts between a tote and backpack plus packs down easily.
http://www.patagonia.com/us/product/...?p=48807-0-497
Get a backpack with two straps. If you load an SLR, spare lens/battery, and water into a one strap bag then troop around for a day, prepare for trading massages to work out the pain. Camera bags have cushy strap pads for a reason.