I bought this Black Diamond Bullet pack a couple of years ago for my wife. It keeps a 100oz Camelbak close to your back and it has just enough room left over for a couple bottles of Super Man along with the requisite day hike essentials.
The best thing about this pack is that it nestles perfectly against the spine and rides like a champ. Add a Nite Ize clip on iPhone case to the shoulder strap and it is perfect. I like it so much I stole it from her and I am not giving it back. I did try to replace it but I can’t find another one anywhere. I am still looking.
Anyway, if you are looking for a good day pack this is the kind of thing I would recommend that you track down. No more, no less.