I always just leave it out and go to the restroom. Maybe some would call that foolish, but I'm not too worried about it. It seems like everyone at the coffee shops I go to already has their own laptop that is better than mine anyways :).

I have asked someone to ask my laptop after sitting with the same people for a while and/or they have asked me to watch theirs - in a kind of tenuous, reciprocal agreement. However, most times I feel it's not worth the effort.