I have often used a small lock like skiers use to lock up their skis, but it is a combination lock, not keyed. But I only use it when I run into a restaurant where the bike is parked in front of a window. My goal is to stop the thief long enough for me to run outside if it looks like a problem is developing. I am not sure what a snowboard leash is (as cited above), but maybe it is the same thing - but I bought my locks before snowboards were invented.

I have never put my bike on a bike rack in the front of a bus, but I have heard of cases where a bus is stopped and someone walking past will grab a bike off the rack and ride off with it. If I did put my bike on a bus rack, I would use something like the skier lock to keep a wheel from turning.