As long as there is still the written word, there will always be editors. Freelance editing and proofreading not only pays a decent hourly wage, it also gives you the chance to read about potentially interesting topics too. What's more, pursuing freelance writing & editing as a business idea can afford you a lifestyle that lets you travel the world as a digital nomad. You can find lots of job postings from companies and individuals in need of writing, proofreading, and editing services on Upwork, which makes this a high-demand side business idea.
Most people, even those many won’t admit it, love moving their bodies. It's also a great workout. If at some point in your life, you had to hang your dancing shoes for a more sensible office job, there's nothing stopping you from earning money with this side business idea as a passion project around your day job. Part-time dance instructors get a fraction of the pie that their full-time peers enjoy, but it can add up to hundreds each week if you're consistent—making for a great side business idea to those with the skills and drive.
If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.