I travel with a backpack and this crossbody bag: Patagonia Lightweight Travel Courier

It weighs 8.2 oz and holds 15L, costs $59. I seldom stuff it as full as it is shown, but you certainly can. It fits my tablet, phone, 3-1-1 bag, glasses, chargers, back up battery, snacks, and a whole lot more. It also holds 2 water bottles. I like it so much I bought another as my everyday carry bag.