Job hunting ;)
In all honesty, I find that when people working full-time jobs look for side projects, either their not happy with the job they have or their job underpays them. (That's not to say that working on fun side projects that happen to lead to revenue is a bad thing, but seeking side project revenue is not a great indicator).
Things that I've heard from friends: - Shopify sites - Consulting - Tutoring / teaching - Affiliate sales