Let someone in line go in front of you.
It can be anywhere.
Most people already let people in line of the a the grocery if they do not have much. This is great, but we can also do it when they do have a lot, but have fussy kids or for someone whom seems to be having a bad day.
Think of nontraditional places where you do not see it much. Like at check in at a hotel or in line for a ballgame. Any opportunity to make someone's day better.