If you have an extra laptop lock lying around, a few modifications can transform it into a perfect secondary bike lock.
Chunkyhampton shares the tutorial on Instructables. It isn't the easiest project in the world, but it shouldn't take too long if you have the necessary materials handy. You'll need to use a hacksaw to cut a small piece of metal tube (a bike frame would be perfect), then feed the end that would go into the laptop's security slot through the tube, and wrap it back around to make a loop. Then just crimp it shut and cover it with some heatshrink or self-amalgamating tape.
As we've mentioned before, cable locks can be snipped pretty easily, so you probably want to use this as a secondary lock in most cases. Still, it's a fairly lightweight deterrent, and worth trying if you have any extra laptop locks handy.
Bike Lock From Laptop Security Cable [Instructables]