Climbing, as in rock climbing? You’re going to need a lot more than a hydration pack, I should think. For one thing, Yosemite Valley in early fall can see just about any kind of weather under the sunfrom 90 degrees to a sudden snow squallso you’ll need some clothing to cover you if it gets cold in a hurry. Plus, well, if you’re really climbing, then how do you carry your rope and so on?
Anyway, at the very least I’d get a small daypack. In most cases, it’s not that hard to add “hydration,” either in the form of a water bladder or a couple of plain ol’ Nalgene bottles. (I’ve said it before and I’ll say it again: I continue to be amazed that there is a market for what are essentially $100 canteens.) Something like The North Face’s Big Shot ($79; www.thenorthface.com) would work fineit has good suspension and twice the capacity, 2,100 cubic inches, of what even BIG hydration packs offer. Even better is the Arc’Teryx Khamsin 38 ($155; www.arcteryx.com), a really nicely designed pack with an excellent suspension. Either of these packs can hold a bladder if you put a little ingenuity into the process. Or, Gregory’s Bang ($79; www.gregorypacks.com) is a funky little town-or-trail pack that has a bladder sleeve and hose port. It’d work just fine.
Yosemite is wonderful. If you’ve never been there before, nothing can quite prepare you for that last turn when the valley heaves into view. If you can, treat yourself to breakfast at the Ahwahnee Lodge.