My kids are younger, so we probably carry more stuff than you need. FWIW, our favorite is actually my kids' LL Bean Deluxe school back packs. They are really sturdy, water resistant, and don't weigh too much. We pack our water bottle in the smaller of the two tall compartments so the cold bottles aren't right on our back.
We've tried a few other bags that were made for travel, but we always go back to the school bags!