Accessing the Debug Menu
Start a new city, go to the Save/Load screen, and choose "End." When the floating moon tells you "Goodbye," pick up the second controller and press Left, A, Right, Y, Up, B, Down, X, Select, Start, Start, Select, R(2), L(2). The Debug menu will now come on the screen.
No Disasters
Using the "No Disasters" option, you will no longer be able to summon disasters. In return, the game will no longer inflict disasters on your city, except in a few special cases: such as an airplane having no place to land and thereby crashing, or a poorly tended nuclear power plant exploding. These disasters can be avoided with good city management, however, and will no longer happen at random.
Needless Money
By choosing "Needless Money," you will no longer have to pay for new structures. You are still limited to structures within your technological age, but once they're invented, you can build them for free.
Valve Max
Your citizens' opinions are important because they influence how many people want to move to your city. You can maximize the opinions of your citizens and thereby have the highest growth rate possible by choosing "Valve Max."
Water Reclaim
Typically, your bulldozer cannot bulldoze water. By choosing "Water Reclaim," you gain the ability to bulldoze anything, anywhere--including the ocean.
Memory
As long as memory is on "Set" then your cheats will be active and you'll be able to access the Debug menu by going to the Save/Load screen and choosing "End." If you want to disable the Debug menu and all its cheats, change the memory to "CLS."