For the most serious of ultralight backpacking adventures, the only suitable options are ultralight, compressible daypacks that stuff into their own internal pockets, taking up about as much space and weight as a t-shirt. These disappear into the pack, yet unfold into full-sized, though often sparse, backpacks for day trips, or even weekend-long adventures.
You'll trade a bit of durability and structure for the weight and size reduction, but many of these packs will last for years even with unpleasant care, and they can effortlessly stuff into a larger pack, so you don't have to wear two packs at the same time as you wander around town.
One of the best overall choices is the ChicoBag Daypack, made from recycled materials, and featuring two side pockets, and an internal pocket, yet weighing about 6 ounces. It's one of the best balances of size, weight, features and price you're likely to find.
Other good choices are the Eagle Creek Packable Daypack, which is a little tougher, but with no water bottle pockets, and the Sea to Summit Ultra-Sil Daypack, which is the absolute lightest pack you'll ever be able to find, at 2.4 ounces.
Patagonia and LL Bean offer more full-featured models for those who want lightweight packs without pushing the boundaries of minimalism too far.
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