Dedicated Hydration Packs
The smallest and least cumbersome hydration pack is designed first and foremost to carry a hydration bladder. Though they come in various sizes, they are smaller than a regular backpack and have limited storage. They may be good for carrying a few small items like keys, sunblock and a spare shirt. These packs are excellent for day hikes and bike rides that don't entail carrying a lot of other equipment. They're a good option for minimalists who seek to keep it as light as possible.
Larger than packs designed solely for hydration, the hydration daypack has the storage space of a typical daypack, allowing for a bit more flexibility. If you're taking a day hike and have extra clothing, food or navigation tools to carry, a hydration daypack is a better option than a small, streamlined hydration pack.
Backpacks with Built-in Hydration
If you're planning an overnight or multi-day trip that requires carrying a lot of gear and supplies, you'll need a backpack. Most backpacks designed for outdoor sports include a built-in sleeve to store your hydration bladder. They may or may not include the actual bladder with purchase, so double-check to see if you'll need to purchase a separate bladder.
Most hydration packs are designed for warm-weather use, since the majority of outdoor sports are performed in the spring and summer. If you intend to use the hydration pack for winter sports like skiing, snowboarding or backcountry snowshoeing, purchase a pack designed for winter use. These packs feature extra insulation including an insulated drinking tube and a bite valve cover that protects against freezing. Kits are also available for adapting a warm-weather hydration pack to winter use.
Wearable Hydration Packs
A newer trend is built-in hydration bladder storage. Camelbak, for instance, has designed garments for both cyclists and winter sports enthusiasts that include built-in bladders. The winter vest is aimed at skiers and snowboarders, designed to keep the bladder closer to the body preventing freezing and making it more comfortable on ski lifts.
A Note About Bladders
Bladders and the packs that hold them come in a number of different sizes. Some common bladder sizes include 50, 70 and 100 ounces. Consider how much time you spend outdoors and how much water you'll need to carry. Also give some consideration to the design of the bladder. Traditional bladders feature a screw-on cap that can make cleaning difficult. Some bladders have a top seal that allows you to put your whole hand in to scrub and possibly even turn the bladder inside-out to clean.