Something else similar to the ultralight daypacks that a few people have mentioned is the Outdoor Research Drycomp Summit Sack. My brother and I strapped them to the racks as a dry bag with tent/thermarest/etc in, and then used it as a daypack for little hikes or just a few days jaunts away from the bikes. Worked great as a backpack, super light, and as a dry bag actually kept things dry too. After being outside for probably about 400 days now it's started to show some wear and a few small holes have developed where the daisy-chain attaches, but overall it was fantastic for on top of a rack.