I compost leftover scraps and food that has expired, instead of throwing it away. In the trash it won't be doing anyone much good since that waste often gets sent to landfills, producing more methane. If it's composted though, you can make an awesome fertilizer to feed the soil and help plants grow strong, without using any chemicals that hurt environment. My mom hasn't needed any fertilizer from a store since we started doing this, and everything has been growing more magnificently than ever in our garden!
I also buy bamboo sheets, instead of paper towels. Being essentially a grass, bamboo will grow back very quickly after it's cut down, so taking this alternative instead of chopping down a tree helps in the fight against deforestation. Another plus is that bamboo uses a lot less water than trees, something we also want to conserve. The fiber in bamboo paper tends to me way more durable on top of that, so you can wash and reuse towelettes made with it, instead of throwing them away, saving money and cutting down on the waste.