You can increase your monthly cash flow in two ways:
- Increase income
- Decrease expenses
It's really that simple. I'd even argue that to a certain extent, decreasing expenses can be more cash-positive than increasing income by the same amount if you're spending post-tax money because increasing income generally increases your taxes.
So if you have a chunk of cash and you want to increase your cash flow, you could decrease debt (like Chris suggested) and it would have the same effect on your monthly cash flow.
Or you could invest in something that pays a dividend or pays interest. There are many options other than real estate, including dividend-paying stocks or funds, CDs, bonds, etc.
To get started you could open an account with any of the major brokerage firms and get suggestions from their financial professionals, usually for free. They'll help you look at the risk/reward aspects of various investments.