Minigunners.
Use the following procedure to sell the indicated buildings for free minigunners. For the GDI missions, sell a Guard Tower or Advanced Guard Tower. For the NOD missions, sell a SAM site or Obelisk Guard Tower (Obelisk of Light). First, select the "Sell" button in the Sidebar. Use the Left Mouse Button to sell the building. Use the Right Mouse Button to activate the selection cursor. Use the Left Mouse Button to select the building and press S. Note: This only works in versions earlier than v1.22p.

Funpark Mode.
Start the game with the c&c funpark command line with the NOD disc. Choose to a new game and select NOD. This will start the game in dinosaur mode. You will play three missions against the dinosaurs and then three other missions where you control the dinosaurs. Note: If you save your game, it will resume in the regular game and not in Funpark mode.

Controlling Air Units.
This trick works for both the GDI and NOD. To control air units in midair, simply give the helicopter a team number. This allows you to give multi strikes without reloading.

Wall Tricks.
This trick requires a Construction Yard, a high enough tech level to at least build Sandbags, and decent amount of money (depending on where your base is located). Build Sandbags/Fences/Concrete Walls to close up entrances to your base. Make sure the walls are not too close to your base, because the enemy will just shoot over it. The enemy AI is too dumb to know that they can "tear" down walls and they will just stop when they get to the wall.

More Tiberium.
Build a "wall" around Tiberium when playing against the CPU. Make sure to also close up the entrances to your base. Then, press R to display the surrender option. Do not press anything else and wait for some length of time. Then, choose the "Cancel" option. The Tiberium amount should have increased on the map. Make sure the Tiberium is "regenerateable".

Train a Free Civilian.
Build a silo, then sell it. One civilian will appear.