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.
When you see a busker don't dash past, pause a while to listen and, if you appreciate the music, throw some decent coins in their hat.
If two people you know are struggling to fund their wedding ask how you can help. It doesn't mean giving them the money, there are many other ways to help by offering your skills for free - photography, cake making, dress making, flower arranging, discounts... the list is endless!