I have a teeny problem with this day. It's not the idea of it, which is wonderful, but the fact it is needed at all. If everyone performed a couple of acts of kindness a week, the world would be a much nicer place.
An act of kindness is anything, no matter how small, which improves the day of someone else. You could top up someone's parking meter or buy two coffees when you only need one and instruct the second be given to a frazzled young mother. You could offer your umbrella to someone who needs it more, or maybe pop to the shops for an infirm neighbour. The possibilities are limitless.