Timbuk2 Hidden Swig. It's almost like pulling a backpack from thin air. This stashable pack is the same size as a regular Swig, but can be tucked away inside its own internal pocket. It's durable enough to carry just about anything, and even has a pocket for your water bottle.
Swig style backpack folds away into insanely small packing pouch
Made from superlight yet ultra durable, 30D Ripstop Nylon