One side hustle I have been doing is Dropshipping. I have been using Shopify for years, for other sites and have gotten familiar with the platform. On the side, I have helped and created myself a few dropshipping websites.
Dropshipping's profit margins tend to be lower, but the cost of starting up is so low. Its easier to test markets using dropshipping, and if takes off you can jump on it full time or look to sell your dropship store to others.
For me, since I'm a graphic designer I create shirt designs and upload them to print-on-demand sites such as Redbubble and Design by Humans. Perhaps I will turn this side hustle into an apparel brand one day, but for now I'm able to earn some royalties as passive income.