The folding lock mentioned earlier is easy to break I would stay away from that one unless you live in a low crime area.

A Kryptonite NY Chain the BullGuard Chain is a good idea because it's usually long enough to go around most things and far stronger than a cable but is quite heavy so some commuters simply leave the lock around the thing they locked their bike too, but some cities may have an ordinance against leaving a lock behind so you'll have to check with your city. Other than a chain or cable you're pretty much screwed if you can't find a small enough thing to lock your bike up to, you may have to search outward to find a place than walk to where you need to go.

All I ever carried was a cable with a Discus lock but I don't live in high crime areas and leave it so I can see it, I also been able to always take my bike into my office when I commuted. I never carry a lock just in case because if I go into a store I just leave it so I can see it and cross gear it.