It's not always all digital when it comes to creating a side business. Why not sell physical products?
I know, I know. Selling shirts/ stuff with prints is a Red Ocean. So what? Find a niche you can sell to and you have the chance to have a decent passive income. Yes, T-Shirt Sales Isn't Profitable, But it is Fun!. And remember that The easiest way to have business ideas is to start a business. Any business.
The first and most important aspect when starting a print-on-demand business is to choose the platform to build on, i.e. to choose a Print on Demand (Fulfillment) Company or a Print on Demand Site. Or in simpler words: do you just need a fulfillment partner behind your own shop or do you need the "all-inclusive" package? You'll find more on this and many more in the excellent The Best Print On Demand Sites for Artists & Designers. This article also covers viewpoints like traffic analysis and trust scores to rate platforms.
Best Print on Demand Websites: The Definitive 2021 Guide adds a consideration of profit margins to the mix. 20 Best Print on Demand Alternative Sites to RedBubble in 2021 gives you even more names in the print-on-demand space.
By the way, you don't have to be a designer to get into this business. There's inspiration everywhere: music, sports, games, politics, comics, movies & series, your friends/ kids/ better half, your favorite subreddit, your side hustle as a maker and so on.