Balzacom has the right approach. Very light daypacks have been around for years. I have used a Lowe "Alpine Attack Summit" for years (Anyone carrying an "Alpine Attack Summit" must be a really gnarly, tough dude, right?)

Despite the silly name, it is very light (3-4 oz), carries a day load very comfortably, and has a lid that protects the interior contents quite nicely. It even has a light daisy chain for technical stuff. On the approach, it is very nice stuff sack for the sleeping bag and/or clothes.

Don't bother with the hydration stuff unless you really like complex, fiddly gear that is heavier than necessary. It's just water. A plain bottle will hold it very well.