When you go for a coffee or meal with a friend, if you can afford to, pick up the whole bill yourself.
If you buy something you wasn't expecting to be in a "buy one, get one free deal", give the free one away.
Whatever pace you are comfortable with, do a sponsored 5k to raise money for a good cause close to your heart.
Turn your computer, and other electronic gadgets where possible, off overnight to save electricity, money and the planet.
If someone forgets to, or cannot, return/repay something they borrowed and, if you can afford to, forgive the debt and never bring it up again.
Take the extra moment to actually put something back on the shelf you got it from if you decide you no longer want to buy it. Get out of the habit of just leaving your unwanted items on a random shelf for someone else to deal with.