I have a 18-liter day pack that can double as a stuff sack for my clothes or sleeping bag. It weighs half a pound and is big enough to carry food, water, some clothes and some other odds and ends. It is comfortable up to an 8 or 9 pound load.

I don't often take it backpacking with me. I regard it as something of a luxury because I can always use my main pack for any day hikes I decide to take away from my base. But when my wife and I backpack together we usually do the 'base camp and day hike' arrangement and then we each bring a small day pack along. Hers is quite similar to mine.