I often carry a fanny pack with me and use it on day hikes from my base camp.

I bought a big one at Wal-Mart that came with two "Nalgene" type water bottles, and I have a small one from LL Bean that was given to me many years ago.

I've seen backpacks that have a detachable part that can be used as a fanny pack too.

If you have to bring climbing gear with you then I have no idea how that'd work for you. Bringing an empty, full-sized daypack might be the way to go. I think my Peak daypack weighs about 3 lbs. That'd be a tough one to want to lug along.