You can sell your ebooks through Amazon's Kindle program or Apple's iTunes Connect, which gives you access to a large majority of the digital-book reading market and the barriers for entry are incredibly low. You don't need to invest lots of money to make this happen, but you do need to invest lots of time, not only in writing the ebooks, but in marketing them as well.

If you’re strapped for cash but want to work on your own schedule, you might want to consider becoming a driver for Uber or Lyft. You can drive as much (or as little) as you want and set your own schedule. Plus, you get the added bonus of having interesting conversations along the way (or at the very least some colorful stories to tell of your travels with strangers).
Know how much content to post. This advice goes both ways. On the one hand, you don’t want to shove unwanted content in someone’s face and turn them away from your platform. But if you don’t post enough, you could be missing out on potential opportunities. So find a good schedule that works for your business -- and for each platform you use -- and stick to it.
If you live near a university, there are likely all sorts of research studies looking for participants. While I was an undergrad at Virginia Tech, I got paid $500 to participate in a 6-week dietary study. The study provided all my meals and paid me, but I had to eat a 5,000 calorie diet of 50% fat for 6 weeks, plus I had multiple muscle biopsies, urine/blood testing, etc.
While this isn’t exactly a long-term solution for making money (since you have to pay it back), it is a reliable way to get some extra cash when you’re in a pinch. It’s also a great way to make money by saving money if you use a personal loan to pay off high-interest debt, such as credit cards. Since getting a loan is one of the easiest ways to make extra money, we felt we had to include it.
This has become a popular business model for online entrepreneurs over the past several years, and will probably just continue to grow in popularity. The best thing about selling online courses is that once you do the up-front work in creating the course and setting up your marketing strategy, you can get paid over and over again for work you do once.