IDLE cash - invest in markets???

[Account Closed]

I have a duplex that has treated me well thus far, knock on wood, and the best part is logging into the checking account and seeing the monthly balance grow (after setting aside for expenses and everything else). 

My question is, does anyone take part of those proceeds and invest in the market in an ETF strategy or something passive (or active)? I'm thinking, $100 in SPY monthly.

Curious to hear what other's do to make use of idle cash that hasn't made its way to the next deal?