What is the best way to earn money on the side?

Sell your unused belongings.
Buy, refurbish and then sell other peoples' belongings for a profit.
Tutor primary school students as the curriculum should be fairly straightforward and the main value-add is being present to guide children on school subjects or on homework.
Turn your hobby into a business.
Start your own website or blog and earn money through selling advertising on your website or blog.
If you are a dog owner, start a dog walking business.