There is a strong argument for hydration packs as the best outdoor innovation of the last few decades. Where once you had to stop, rummage around in your backpack for a water bottle, uncap it and take a drink every time you felt thirsty, all a hiker with a hydration bladder in their pack has to do is flip the cover off the bite valve and take a sip. There is no need to stop. The benefits for hikers, cyclists, horse riders, climbers and runners are easy to imagine. Many people find that they get through a lot more water with a system like this, and as a consequence, they stay more hydrated and feel less tired at the end of the day.
Klean Kanteen Stainless Unibody Bamboo Cap KCSSLB-BS
Camelbak Rain Cover S/m 60113
Camelbak Arete 18 Pack 62283
Some bags are designed just for hydration but most modern day packs and a many expedition-sized backpacks are built with pockets intended for hydration bags. Just fill the bag, pop it into the pocket, and push the tube and valve down through the outlet hole. There is often a clip or an elasticated band on one shoulder strap to keep the tube in place. These are always worth using, especially when it's windy. They'll stop the tube from flapping around and make sure that the mouthpiece is within easy to reach when you need it.