I don't like to carry the heavy U-lock that I use around town on longer trips, but I am not confident enough to go without any lock. I carry a cable, and my bike has a ring lock that I put the cable through. Sometimes I secure my bike to something overnight if I'm paranoid. Sometimes not. And sometimes I feel perfectly comfortable leaving my bike unlocked. With the ring lock, I also have the option of just engaging the lock and not worrying about the cable. It won't stop someone from throwing it in their vehicle and driving off, but it will stop someone from hopping on and riding off.
Of course the downside to any key lock: my last trip I got all packed up and ready to load the bike one morning and could not find the key. I was weighing my various options such as finding a hacksaw or having my wife, who I was supposed to meet at the end of the day, come fetch me with a spare key. Then I found it in yesterday's pockets.