I just bought a cable lock for my upcoming trip. I don't plan to leave my bike locked up anywhere for a long time where lots of people will have access to it. And, even if I did, even the most sturdy locks are only a deterrent to an accomplished thief. With that in mind, I opted for something that is strong and sturdy enough to deter an opportunistic would-be thief, while still being relatively light in weight. I ended up with an On Guard cable lock with keys from REI. I think it's the .39 in or .41 in one or something like that. It weighs about 14 ounces.

Also, regarding combination locks vs keyed ones, combination locks are supposed to be pretty easy to pick. Mine came with a bunch of keys and my friend and I will both have locks, so we're taking spare keys for each other.