Online courses. If you upload online courses on a platform like Udemy, people will be able to discover them every single month for years to come and you'll get paid a percentage of every sale the platform makes;
SEO-optimized websites. If you create a website with useful content that people search for on the search engines, you'll receive free traffic every month and you can monetize it by simply having some banner ads that other companies (e.g. Google AdSense) run on your website. These days it's not that easy to get good rankings on Google and other search engines as there's so much competition in the market, but it's possible;
Amazon e-books. If you write e-books and publish them on Amazon, people will be able to discover and purchase them for a very long time. I know that Amazon's search engine generally prefers new books to old ones, so you might not sell as many books as you will in the first 6 months, but you'll still make passive income every month from the books you wrote many years ago.