The best way to earn passive income is through investing hands down. Personally I have invested in S&P 500 and Dow Jones ETFs. I started when I was 22 and have made more from it than I have playing poker. You basically sit and make money. It isn't without risk and I am in it for the long term. I opened up a Roth IRA account and a brokerage account. Your money grows exponentially this way. Never underestimate the power of exponential growth! If only I had invested when I was a teenager. Man. My Roth is more of my safe investment. My more risky ones are things that I speculate.
My newest venture is into the crypto world, namely bitcoin in particular. I'm with jfound. Bitcoin is truly revolutionary and the blockchain technology will change the world. It is the future.
To supplement both my desire for poker and bitcoin I've joined Betcoin poker. Basically, you play poker, but instead of using the dollar you use bitcoin. In a way, you are investing with your investment. You play poker and actively gain more bitcoin, while at the same time your bitcoin increases in value while you are sleeping.
I'm so passionate about bitcoin that I've even built a website telling people all the different ways to earn bitcoin. It is my profile picture too.
http://www.earnthosebitcoin.com
Some other ways to earn passive income are affiliate marketing which I have tried countless times and have not been very successful at. I even taught myself HTML, CSS, PHP, MySQL, and other things. Affiliate marketing is tough though so it will cut into your time. If you are successful though, then a lot of the things you've built will work for you.
The easiest way to make money sitting on your ass doing absolutely nothing is to invest in stocks and hold them for the long term. By long term I mean at least a year, perhaps several years. Preferably 10+ or 20+ years.