For a quick trip into a grocery store or a restaurant, I just use a skier type lock, very fast and very light. In restaurants, I try to find a seat where I can see my bike. Skier lock below, tiny, easy to use, retractable cable makes it very convenient.
Also bring a cable lock for when I am in campgrounds, or in areas where I worry more about theft. A skier lock will only slow a thief by less than a minute, the cable lock is less convenient to use, but it slows a thief for several minutes.