The biggest problem with using backpacks in the summer is the uncomfortable sweatiness. Gregory's latest mid-size day pack, the Z 22, features a unique suspension system that they call the "Jet Stream Dynamic Transfer System (DTS)" to prop the pack slightly off your back. The DTS system allows air to circulate between the pack body and your back, keeping you cooler and cutting down on sweat stains.
An exoskeleton-like frame (usually only found in larger packs) maintains the bag's shape and the DTS suspension adjusts itself in response to how heavy your pack is. This keeps weight distributed evenly, making for a much more ergonomic experience.
Made with ultra-light materials like ripstop nylon, I like its slimmed-down design which gives it some urban appeal. I can do without the hip-belt stash pockets for use in a city, but they'd be helpful in the backcountry for easy access to important tools. It's a versatile pack, fitting in well in urban or wooded environments.
Available from one of these vendors for $99.