Investing rental income stream

[Steve Miller]

Looking for thoughts on or ways that REI's use for leveraging their rental income stream. I understand rent first goes to the mortgage, then the management of the property and its bills. I also believe in having the rainy day fund accumulate for the eventual repairs and larger lifecycle replacements. With that $100 or more per door, what do you do with that money while it sits? Invest in low to moderate risk mutual funds so its readily available?

I'm interested in ways to grow that income while it sits but also have it available when the need arises. Even if I accumulate that revenue for my next purchase, I would want to get some additional return on that.  Thoughts?       Thank you!