1. Hold the door open for a stranger.
2. Tape a buck or two to a vending machine.
If you go to school or work at a place with a vending machine, taping a dollar to the window means someone can enjoy a nice treat that now tastes EXTRA great.
3. Leave quarters at the laundromat.
4. Give your compliments to the chef (or the manager).
5. Buy someone's coffee.
6. Make a kit.
Take a sturdy plastic bag and fill it with toiletries and such -- tissues, toothpaste, a toothbrush, nail clippers, new socks, etc. -- and leave it, with a note saying it's up for grabs, at or near a homeless shelter. You can also leave some basic goods (socks and things like tampons and pads are especially needed) directly at a local shelter.
7. Be a meter fairy.
8. Bake something.
9. Say good morning.
10. Leave a note.
Leave a Post-It with a nice message on a random spot -- a stranger's locker, maybe, on a library shelf, or in a restaurant bathroom. It's a simple and unobtrusive way of paying a compliment and making someone smile.