Deuter AC Lite 20

This is a great pack for day hiking. I used it in hikes up to 15 miles and was able to fit all I needed for the day, including water. Very comfortable, stable, light weight, and great back ventilation (thanks to the back mesh design).

My only minor complaint, the side mesh pockets are not tight enough to hold a large water bottle (they should have added a strap to secure the bottle), but since I use it with a hydration bladder, it's not an issue for me.
This is my favorite day hiking bag.

This is far and away the best daypack I've used. Everything I need fits for 10-20 mile hikes, both hot and cold weather.

Unlike another reviewer, I have no trouble carrying large water bottles in the mesh side pockets. Very comfortable in all conditions. The mesh back is particularly welcome in hot weather.