I personally am not keen on having a key to lose, so prefer a simple coil lock that isnt too heavy, packs small yet can still get around a tree etc. Coil locks tend to have plastic on them so they won't scratch your frame. As you say, these are situations where we are really stopping opportunistic thefts.
+1 - this is what I use, it weighs a little less than a pound, and will stop all but a dedicated thief. I hang it off one of the buckles on my front panniers so it's easy to get at when ever I stop. I'm paranoid about bike theft, though, I lock up my bike when ever I step way, even if I can see it out the window.
I love the idea of a snow board or ski lock for how light and easy they are to use, but I think that someone could bust it with a really good pull.