Hydra Packs are a MUST for hiking! There are many brands and sizes available (no endorsement funds coming in for this promo). I like this Camelbak Cloud Walker 18 because it's light, comfortable, has extra room for snacks, great pockets, and a 2.5 liter reservoir!
For a lighter pack, I use my cycling hydra, the Camelbak Aurora. It it smaller and still holds 2.5 liters.
Tip: Get an insulated tube so your water stays cool in transit. :+)