We have quite a few cheats for Majora's Mask! Check 'em out!

BREAK INTO SAKON'S HIDEOUT
Go to Ikana Canyon up to Sakon's hideout. There is a big metal door on it that can only be opened by Sakon. So, you have to have Sakon open it for you, right? Wrong! Go up to the sign on the right side of the door labeled "Sakon's Hideout: Impenetrable Security" or something like that. Notice the crack near where the door meets the rock wall. Roll at that crack. If done right, you will roll right into Sakon's hideout! If done wrong, you will either roll into a void where you fall and hear a splashing noise or you will just bounce backward. This might take a few times to accomplish and if you go in on any day other then the third, there will be no one there.

WEAR THE FIERCE DIETY'S MASK IN CLOCK TOWN
Go to Ikana Canyon on Dawn of the Final Day and play the inverted Song of Time. This way, you will have more time to play around as Oni Link. Anyhow, go into Sakon's hideout (using the Sakon's Hideout cheat above). Be sure to go in wearing any NORMAL mask. When you're controlling Kafei, switch the normal mask with the Feirce Diety's Mask and when you're controlling Link again, you will be Oni Link! Once out of Sakon's Hideout, swim down the river that leads to the Southern Swamp. From there, you can go to Clock Town! Once in Clock Town, the guards will treat you like you are a kid and if you talk to a Bomber Kid, the game will freeze!

RIDE EPONA IN CLOCK TOWN
Wait until almost 6:00 at the end of any day and get on Epona in Termina Field. Now, run at one of the entrances to Clock Town as if you were going in there. Just keep running until it says "Dawn of the 'Whatever' Day" and when the screen comes back, you should be in Clock Town with Epona!

*I never confirmed this one when it was given to me, so I went ahead and put it up, but after many tries, I've come to a pretty solid conclusion that this one does not work. But, if you're bored, it gives you something to mess around with.*
ESCAPE CLOCK TOWN AS A DEKU
Buy the Deku Flower in Clock Town from the Deku Scrub salesman for the Moon's Tear. After the Deku Scrub flies away, jump out of the flower. Fly over to the exit to Termina Field and you will be able to fly right over the guard's head! It won't do you any good though because you will wind up in a void of no escape.