In the ghost house, hit the first P switch, and run as fast as you can to the right until you reach the last door (and a free extra life). Go in, but do not hit the P switch yet. Carry it over the tempting coin pyramid, and jump into what appears to be a dead-end (except the door that goes back to the beginning). Hit the question mark box and create a staircase of coins by directing them with the D-pad. You do not have to hold it down -- they will keep going in the last given direction until they hit a wall or coins. After you have made a staircase with them, hit the P switch, and climb up quickly to find a secret key. It also helps if you are small, but it is possible to get in there when you are big -- just duck and jump.

On the first ghost house (not the secret one), get a feather and run to the right. Then run back to the left where you began and fly up. Run to the right to the secret key, and you will be taken to the Top Secret Area.