DH and I spent waaayy to much time looking for "the perfect bag" for our trip last summer. We both wanted something either of us could use, be water repellent, and light weight. We ended up with the Stowaway Pack Backpack from LLBean. It was PERFECT. Lightweight, the straps are padded put have holes in them so they are breathable, it poured two or three times and everything stayed dry inside. I actually convinced my parents to buy one for their next trip (they've been using the same purple backpack for 10 years now). Here's the link to it (I'm hoping it works!)
http://www.llbean.com/llb/shop/63707?feat=816-CL1 I'm glad I didn't get a "girly" one DH ended up carrying it most of the time.