JoJo Designs' Carrie Laptop Bag is the kind of summery laptop bag I need; in lightweight canvas, it would be light enough to toss my laptop into, along with sunny day essentials like my sunglasses, water, etc.

Its lightness belies its durability — just check out the cross-section photo of the bag loaded up with computer, books, and everything; of course, in the event that you do overload, they include a handy-dandy optional shoulder-strap.