Build something to solve your own problem.
Last time i did (wrote real time data backup and mirroring tool) - it brought about $200k in "side cash" :)
There are tons of opportunities out there. Checkout indiehackers.com for some ideas. OppsDaily newsletter is another good one. They send out daily problems that people are willing to pay for.
Write a book. I pull in an extra $2,000/month from https://secretsaucenow.com
This blog post: http://ryanluedecke.com/ideas-for-startup/ has a ton of useful info, especially idea #14 which explains how you can turn popular “how to” blog posts into a service or product.
That's partially what I did for my https://citybreakflights.com service
Nugget.one has a daily idea. Note: you're going to get broad answers. If we had some details about your experience or hobbies or skills... you might get some fantastic suggestions.
Learn how to invest your surplus dollars and start saving for financial independence. A simple Warren Buffet blessed approach is 90% VOO and 10% BND. Save 25 times your annual spending and never work again. Think of your extra dollars as employees and put them to work for you.
http://www.mrmoneymustache.com/2012/01/13/the-shockingly-sim...