When in MS-DOS or DOS go in the C&C directory and type in "C&C FunPark". This cheat gives you hidden levels with dinosaurs and barbed wire in 2 Player mode.
Sell Units - Part 1:
Send the guy or car right next to a sandbag wall and then click the sell button. The $ icon will come up. Put the $ cursor as close as the unit as you can without it becoming crossed out, then click to sell. Voila!
Sell Units - Part 2:
If you are short on money or don't want the tank just put it on the repair bay. Then select the Sell button and move it on top of the tank. The cursor should turn into a green "sell" cursor. Then you click and it sells the tank. This also works for Orcas, Apaches, harvesters, APCs, and APCs full of infantry units.
Build a turret, obelisk, or tower. Sell the building, but cancel the sale before it disappears. You get a few minigunners every time.
When you have more than 1000 credits, buy a silo. When it gets more than three squares filled up sell it. You will not only get money for selling it, but also for the stuff inside it. Can be repeated as many times as you like.
After building Com. Center, click on a moveable unit. Then build a structure, but don't place it. Scroll the screen so there is a building with enough cells on its right side to place the structure you have built, with one cell of that space separating the placed building and the map. Click to place the structure and move it to the space next to the map.You should see white hash marks. Then move the cursor onto the map, where it should show as a green 'move' cursor. Click on the map where you want to place the structure...
If you're having trouble versus the computer and have some extra money, just expand with sand bags. Move towards the enemy base and sell the bags behind you. When you have reached the enemy base(s), just put a 'cork in the hole' by putting sand bags in the entrance of the base. Now the computer is trapped, unable to do anything. Gain control, build a devastating army, raise some towers in front of the base to weaken the enemy, put sand bags over the destroyed enemy towers' spots, just kick ass. It works every time!