I have a Vaude Trial Light 12 is just enough for essentials, a shell, a couple water bottles and a snack. It is an older model with water bottle pockets on the outside, a feature that I really like and has been eliminated from the current model. I adapted the waist belt to make it removable but cutting the straps off, leaving 2" and sewed 1" ladder locks in place so a belt can be added. After using it for a while, I probably wouldn't bother. It also makes a great urban pack with room for a rain shell and a camera, but is small enough that it is easy to maneuver doorways, buses and crowds.
I just got an REI FLash 18 and I'm surprised how well it carries. I wish it had water bottle pockets. 18 liters is plenty for a day hike if you are sticking to basics. It had good room for a puffy layer, a rain shell, food, water and essentials. It isn't big or stout enough for carrying a DSLR kit or other bulky/heavy toys.
For a no-compromise day pack, I use an old-style Osprey Stratos 24 (http://www.moontrail.com/osprey-stratos-24.php). It isn't a lightweight at 3 pounds, but I can throw the kitchen sink in there and carry it comfortably. It is a "trail office." I had a Talon 22, which is a fine pack, but it wasn't quite big enough and the side pockets were too small. The Stratos has large side pockets that will handle a point and shoot camera or a small GPS, a well as snacks, insect repellent and sunscreen. The back panel is a trampoline style and a joy on a hot day. The pack is fully framed and gives great weigh transfer. That trampoline frame creates a curve in the pack, making it appear larger capacity than it is, but the water bladder rides in there, reclaiming some space and isolating the water from the electronics in the pack. You can remove and replace the bladder without unloading the pack. I don't like the water bottle pockets-- they are too small for typical one liter bottles and they will poke me in the ribs. I found that recycled springwater bottles with tapered tops will clear my ribcage. The side packets are stretch mesh and I've found them handy for a bandana and gloves. There is a bog stretch pocket on the front that I like. I keep a Z-seat CCF foam sit pad and my maps in there. I could do a summer overnight trip with this one and SUL gear--- SUL for compactness as much as light weight.