- Credit Card Arbitrage,
- Fixed Deposit
- and 40 and 100 and 200 more such these kind of ideas.
Of course, just like every original idea of the world, it's always the copycats who spoil it. When you go Google "Passive Income", there is literal 1000 web sites which discuss various passive income idea.
As always, boredom is the real enemy human being wants to fight.