I've tried a few things over the years... most as side-gigs to see if I could make them full-time jobs.

1) Taking what you know, and doing it at a smaller scale. Instead of building eCommerce sites for Fortune-50 companies, doing it for local Mom & Pop Shops... Outcome : Meh, I spent too much time looking for customers, and too few hours billing for work. The projects were just too small. It may work, if you can some how get enough small customers, but I found it really unfulfilling to have to "cut corners" to meet the really tiny budgets that small customers have. And working with things like some shitty junk template for Shopify or WordPress Commerce was just torture.

EDIT : I also lost about $85k on a $55k project trying to do something for a small business. It was embarrassing. I took on a project that was a bit too big for me to do all alone, so I contracted a team of devs that a friend raved about... and they didn't work for me. Anyway ended up having to pay them, and just lost 6-months of time, totally embarrassed myself to the customer (who I had gotten through a friend), and in the end I ended up doing like 5x more work than I had planned and hiring a (non-cheap) friend to come in and help save the day. I don't know the advice I learned here really... other than it just flat out sucks to have things go sour when doing a side-project that you're personally on the hook for delivering. In my case, it came down to me trusting the wrong people. Really is quite valuable to know the people you are working with.

2) Building stuff outside of tech. I built a few tables and benches and such for friends in my garage. I posted a few pics online and found a buyer for an item, and like I went in thinking I was charging a fair price. Outcome : I like woodworking, so the work was fun. The downside is that these sorts of customers are a pain in the ass. Guy commissioned an "indoor table" and then put it outside space... and 3 months later he was mad that the finish wasn't holding up. In the end I burned a lot of time, basically had to refund the entire sale price, lost money and time trying to remedy the situation (not to mention haul the item to and from my garage shop multiple times), and just got really frustrated with humanity. Like it's one thing to do some tech work and have people trash it... it's a whole other thing to have someone take a beautifully finished indoor table and then stick it out in the rain and cold for 3 months and then be upset that the wood warped and want you to burn hours fixing it... Plus this customer was just such a pain in the ass. And you have to "love your customers" to get started in this sort of space, and love engaging with Social Media.

3) "Coaching" / Project Governance. I found some people who would have hired me to do work, but instead had hired some (cheaper) shops. Then I found a role overseeing the work and coaching them on what to look for and what to expect. Outcome : Made good hourly money, but in the end it was way too similar to my day job... only with a lot more headache. Software teams that were in India, or other places that were just the most god awful teams you could imagine. Bad quality. Bad communication. Dishonest. Shitty shitty off-shore hours. But also clients just kinda sucked. Marketing folks who would sit and listen and give praise for advice, and then disregard it all and then come back with, "Oh thanks for your good advice last time, let's say I didn't listen to any of it and all the bad things you said would happen happened... now what do I do to save the project?" But, of the 3 options so far, this was what paid the best. I did this a handful of times and some were OK, but most were really frustrating.

4) Angel Investor. I found some small local businesses... one in tech, the others not so much... and just got involved. Helped with stupid things like marketing sites, or setting up POS systems... whatever I could help with. Gave them a bit of seed money -- either as a loan, or purchased 10% of their business. Outcome : Lost money and time on 3 businesses, but one of the companies did OK, and I could probably live off the returns for a while if I needed to (or sell my share). It's hard to predict which companies will do well, but being an Angel Investor was nice in that you get enough control (people don't just listen to you and ignore you), and you get some hands-on work, and the returns can be pretty solid. The company that did well was just something dumb... a neighbor's friend was building a few warehouses to rent to Amazon.