Making Your Camera Backpack Hiking/Trekking/Hydration Friendly.
I did this with the Lowepro Flipside 300. As long as you have a few attachment places, you should be able to replicate this process with any number of bags. All the pictures below and additional ones are in this set on flickr
I love the Flipside, and it nearly always fits my needs. The problem I run into is lack of hydration. I bought a long thin water bottle by Klean Kanteen which fits the pocket and works for normal city type outings. However, when I want to bring my camera on a hike, I have to decide whether to stuff the camera into a non-camera backpack which has a hydration reservoir, or tough it out without enough water. I like neither of those options! I went to our local camping store, REI, and cornered a man (who, in retrospect was probably on his break!) and he and I worked out this solution.
Attach the Camelbak to the backpack using small bungee cords and keychain caribiners.
Flipside 300 ($60-75 depending on color and vendor)
Camelbak Stowaway 100 fl. oz. Reservoir $40 http://www.rei.com/product/791564
Coghlan's Mini Stretch Cords $2 (4 in a pack!) http://www.rei.com/product/674921
Nite Ize S-Biner Size 3 $3 each, need 2 http://www.rei.com/product/771122
REI Duck's Back 20 Liter Rain Cover $20 http://www.rei.com/product/778474
(Not required for adding the reservoir, but something I wanted to have on hand since the smaller Flipsides don't have an all-weather cover built in. This size is large enough to cover the backpack and a small tripod/monopod. It fits better if you remove the ballhead.)
Total USD $128-143 including the hydration reservoir. This compares favorably to the $180 Clik Elite Escape backpack which has a reservoir space, but does not actually include the reservoir! I'm sure for regular abuse, the bags designated for rough hiking activities are better, but for infrequent use, this works well.
How to attach the hydration reservoir:
1. Attach a keychain caribiner to the loop of the waistbelt, behind the plastic adjuster toward the backpack itself, so that it will not move much. Repeat on the other side.
2. The Camelbak Stowaway has an adjustable loop at the top, attach this to the carrying handle of the Flipside bag.
3. Attach one Mini Stretch cord to one of the keychain 'biners, send it through both D-rings on the Camelbak Stowaway and attach to the 'biner on the other side.
4. For just a little more added support, send another mini stretch cord through the hole for the tripod mounting strap, around the entire Camelbak, and attach each end to itself.
I would recommend carrying an extra set of the mini-stretch cords or having spare rope/shoe laces in case of breakage during a trip. Most of the weight-bearing occurs at the top of the bag where the Camelbak's loop attaches to the Flipside carrying handle, and the stretch cords are mostly to keep the attachment from flapping around everywhere.
Overall, this set up is nice because the items are modular. I can reduce weight by leaving home unnecessary items and items can also be used on other bags. It is easy to replace broken/chewed parts. It might look a little strange, but it definitely works and saves some money on the more dedicated bags.
I can carry the monopod under the hydration reservoir if I want to, however, this is a little unwieldy when everything is full to capacity. Plus, that's a lot of stuff to undo before the monopod is accessible. I would do this for a longer hike when I know that I won't want the long lens and monopod until I get to a particular destination, otherwise, I would just carry the monopod in hand (or pass it along to someone else!)