I've been using the Deuter Race EXP Air for the last two years and love it! It's a hydration pack, but I removed the bladder for commuting and am able to fit my 15" Macbook Pro, iPad Air, tupperware, shorts and sunglass case with "a little" room to spare. It has a built-in rain cover stored at the bottom of the pack and if I need to carry anything extra, I'll use the helmet straps to hold it on the outside of the pack.

Here's a link to the pack.