You all know this, but replacing the air filter is the cheapest way to improve the air quality in your home, removing funky smells and allergens. And you can get a 6-pack for between $80 - $100 today, which is about 15% less than usual for most sizes. Buying these in bulk might encourage you to change them more regularly, which puts less strain on your AC unit. Good for you, and good for the environment. Win, win!