I'm not sure there is such a thing as completely passive income. Wealthy investors (think China, Saudi Arabia, and Japan) have a lot of money (say 70 trillion) and thousands of people trying to allocate it efficiently. They don't do so well. Why would you?

Also, my understanding is that real interests rates are currently negative. On top of it all, the combination of a nominal capital gains tax and inflation means you lose ((inflation * .15) * your net worth) every year in this manner, at least if you are a US citizen. In light of all this, perhaps you should consider spending money on your intellect; it's not taxed, and it has some degree of liquidity. Alternatively, buy a cheap foreclosed house and rent it out; that may be profitable right now.