If you run an online blog that is focused on a particular topic or niche, then publishing an ebook could be a fantastic way to make money. Sites like Nexus Notes or Stuvia let you upload your notes to sell. They take a small percentage but you can make lots. Make sure your notes are typed up and not handwritten!
Renting out your home or even a tiny corner of your garden is an excellent opportunity to earn some extra money. You can advertise on websites like Airbnb or via word of mouth and local classifieds.
If you've got a knack for design and logos, then logo design is another way to make money online. Just be careful that it doesn't take over your entire life!