Find a stranger, talk to them for a little, if they seem really nice and pleasant then pass them a $20. Money always makes people smile. Or when you go to fast food, buy the person's food behind you. Or do the same with coffee. Or just give a waitress a nice tip.
Donate to the Red Cross.
If you see them in a store, ask him/her if he/she has someone special they were planning to buy a gift for, then offer to pay for that present (if it is something you feel you can afford).
Give a homeless person the coat/jacket of your back
How about a random donation to a charity in their name?