I think for a lunch stop what you need is pretty minimal. Depending on the neighborhood of course, but our groups only sporadically lock bikes for a breakfast or lunch break. It's actually more common that we don't.

I actually feel more comfortable leaving my road bike unlocked than I do my commuter - just for the simple fact that, given the clipless pedals, some kid isn't going to be able to see an opportunity and just ride off with it.

I've considered the same question, and so far I haven't bought a lock for the road bike, but I'm thinking along the lines of a very light coiling cable. When I'm in a restaurant - looking out the window at my bike - and many people are walking in and out in view of wherever the bikes are parked - all I really need to do is create a teensy disincentive or obstacle. If someone backs their van up to my bike I'm going to know about it - if someone pulls out a bolt cutter I'm going to know about it - and I'll have enough time to get out and confront them.