I use a Kryptonite cable lock. But, it's only for quick bathroom breaks. I never, ever, leave my bike unattended for more than a couple of minutes. Which, of course, is plenty of time for someone to clip the cable and take it. But, it at least prevents someone from just grabbing it, and riding off. If I stop to eat, it's either a place that allows me to bring it inside, or a place where I can lock it with a window, so I can keep an eye on it.