I'm surprised that bug bounties haven't been mentioned yet. Sites like HackerOne and BugCrowd allow you to use (and improve) your skills while also potentially

I'm surprised that bug bounties haven't been mentioned yet. Sites like HackerOne and BugCrowd allow you to use (and improve) your skills while also potentially making some money while doing it. While HackerOne has a lot of web bounties there are a few mobile and desktop application bounties as well.