I use the above pack for almost all my outdoor activities, and it works great! It cost me almost $80, but I noticed it on sale here for much less:
http://www.prolithic.com/hpages/trn_acc/ultcycle.html
It holds a good amount of water, my first aid kit, snacks, knife, etc., plus trading items for caches. I can stuff a jacket in there if I need to.
The best thing is that it has a great suspension system and hugs my body well. I did not like the hydration bladder that it came with, however, so I replaced it with an Eddie Bauer one from Target...no big deal.
There is a photo of the pack at the above link.
Scott
Lynchburg, Virginia