I use a 30L Timbuk2 backpack that I got on sale and it's been going strong for 4 years now. Not 100% waterproof but it is pretty water resistant and holds a lot more stuff than you'd expect just by looking at it. I'd recommend any of their packs for durability and...
"The tree never saw me coming." Didn't smell or hear you, too! Great report - I'm going to Idaho this year and I guess won't be able to rely on Google Earth as much as I thought I would.