Holding open the door for someone. An age old tradition. Gentlemen do it for their dates, boy scouts do it for old ladies, and if you're in public, people are generally expected to do it for each other.
I prefer to hold the door for someone far away, for an obscenely long time, making them nervous and often times causing them to sprint for the open door out of some sort of social obligation to keep you from holding it too long, only to slam it suddenly shut. Now they're out of breath from running over, and they still have to open their own door. Today I got some poor sucker so bad he literally stood there staring at the door for a good five seconds. That that, ya jerk.