I like the look of that Kelty sling pack. For almost any hike of distance I also carry an Outdoor Research summit sack, http://www.outdoorresearch.com/en/or-gear/drycomp-summit-sack-chili-1size-us.html, waterproof, not bulky and light enough to go in another bag. I use it for shorter hikes where my full pack is extraneous, and filling with clothing etc when I want to make doubly sure they stay dry. I find it best for bulky but not heavy items, it doesn't have nearly as much padding or suspension support as a regular pack, but you can sling the summit sack over your other pack, or carry it for a while as a chest-carry sack - although that works best for shorter distances, not a whole hike: situations such as you're toting some gear for a hapless younger Scout who overpacked.