Almost every engineer I know has dabbled in freelancing. It's a bit like a drug. Easy to get started. The extra cash can be kind of addicting. Irritability after you stop and you're back to your regular salary. 
Let's explore some other ways of making money as an engineer without trading time for money.
 

1. Create A Course 

Time to monetization: Short
This is by far my least favorite. There's a lot of people doing this, it's too easy to get started. There are services that are helpful though such as https://gumroad.com/ that will do most of the heavy development for you so you can get started right away. You'll have to do most of the marketing yourself but you'll be able to just focus on the course creation and your brand

2. Sell Code

Time to monetization: Short
Instead of starting a company, or building a product, you could make a useful module, component, npm package, theme, or any piece of software and sell it on SugarKubes. You can write and keep your code on GitHub and monetize a private GitHub repo directly.

3. Sell an API

Time to monetization: Medium
Through a site like Rapid API you could write a lambda or a backend and start monetizing it right away. They have a ton of APIs to choose from so make sure you find some open space to insert your product and get started very quickly.

4. Sell an AMI on Amazon Marketplace

Time to monetization: Medium
If your project is more involved than just an API, you might want to consider selling a pre-packaged machine image on Amazon's marketplace. You'll be able to add hourly fees for running the machines on top of what AWS charges for compute and it charges your customers directly from their existing AWS account so they don't have to set up an extra billing site. I haven't been able to get a hard number from Amazon upon request for exactly how much these AMIs drive revenue for the sellers but my suspicion is that it's relatively new and underutilized. The upside is that when people turn these things on, they either forget they're on or get so used to the convenience it's easy to just keep using them.

5. Create A Product 

Time to monetization: Long
Obviously, this is the hardest one. If you're going to go this route, perhaps try a no-code version first. As developers, we're far to trigger happy to pop open an empty text editor and start everything from scratch. 

6. Advising 

Time to monetization: Long
This is a good deal if you can find it. You're going to have a difficult time sourcing projects early enough. By the time the company has taken institutional seed money, you're going to have to jump through more hoops to reach the founders and prove to them it's worth their time to take your call. If you haven't had an exit, or haven't advised before, I'd recommend getting to know a few founders and just making yourself indispensable as an advisor. The good news is that many institutional investors don't spend a ton of time on each company. While it's true that they're time and connections can provide immensely helpful, there is still a plethora of work that requires good ole' fashion elbow grease. Start with those tasks and it will be easy to be a value-add advisor not taking a salary and not asking for any equity yet. If the company starts to grow and starts to raise their initial seed round, at that point you've earned the ask to continue to be an advisor. 

Bonus: Investing 

Time to monetization: Long
Expect time horizons in the 6 to 10 year range. Companies are staying private longer and are thus not liquid for longer. There are ways to free up cash like https://equityzen.com but of course, you'll have to get approval from the founders, or the board, and good luck with that. Most companies don't want a cluttered cap table. It can confuse and annoy future investors. A messy cap table may impact a company's ability to raise in the future.