At lunch today, see what you can do to brighten someone else's day. It might seem odd for me to zoom in on lunchtime, but we should start somewhere, right?
Do you eat at home alone? Consider inviting someone over to join you at mealtime. Another alternative to that would be to go out somewhere, but still invite a friend. You don't necessarily need to volunteer to pick up the check. Just giving someone company can brighten their day - and yours!
If you normally go for take-out, think about buying an extra meal (a simple sandwich for example) and then handing it to a homeless person on the street. Standing in line at a fast food place? Pay for the person behind you too. (Since you don't know what they are ordering, approximate the cost or just casually ask them).
Having lunch at work without going out can be a bigger challenge. If a coworker approaches the microwave at the same moment you do, insist that they use it first. If you have fresh fruit, cookies, crackers, or anything easily shared, offer some to a coworker.
These are just a few examples. I'm sure if you think about it you can come up with many more ways to brighten people's day at lunchtime.