The easiest way to make passive income is to invest in a long term. Why, because you just wait and wait. Until the day come you will earn just by waiting.
When they say passive it means an income that keeps flowing every month or every quarter. One example of this is the NEO token by which if you hold on to it in your wallet you get GAS every distribution that means a passive income. If you have a token that allows you to set up masternodes then that can also be a good choice, there are tons of these that you can pick.