I was building up a local rail and road transportation network then on a whim I built a few airports and setup some plane routes between them. The revenue from planes started outpacing my existing ground networks quickly. I then built two large airports on opposite ends of the map and purchased a large jet to fly between them. Before long I had more money than I knew what to do with. A long distance air route between two large cities with a fast, high capacity jet just prints money.
So I'm wondering, how do I get some more challenge out of this game? What are some more fun things I can do? At this point I am making money faster than I can spend it.