Yup, after a certain point (the game will tell you), you can purchase businesses. Once you've purchased the business, you can step into a pink marker, which will start a mission for that business. Once you've done all the missions for that business, you will have completed that asset (it will tell you this), and it will now start generating money for you. The Malibu Club makes the most money at $10000 a day.
But imo the fastest and easiest way of making money is through the Vigilante missions using the brown Hunter helicopter. You unlock this helicopter by collecting hidden packages (I think, or it may already be available at the Army base). Within an hour, I made over 200,000,000 dollars. Yes, 200 million dollars
