Normally, you'll need to use Mario's 30th Anniversary Edition amiibo if you want to play the game with a special effect applies that causes the game to look as if it is being played on an old CRT television. However, you can also activate the effect by holding DOWN on the d-pad while a level is loading and pressing A+B. If you did it properly, the effect will take place. You can disable the effect the next time you load a new level, in the same manner.
Playing Near Doors
In Edit mode, place a door and tap on it with your stylus. You should hear a knock in response, from the other side. Listen for the number of knocks, then reply with the same number. The process should repeat a few times, and as long as you continue responding correctly for long enough, you eventually will see the door open and a character will appear.
Skinny Mario Mushroom
Find the Nintendo World Championship Levels and clear all of them. From there, head to the Level Creator mode. You'll find that you can now shake the original mushroom to produce the skinny mushroom on demand.
Nintendo World Championships
At the 2015 Nintendo World Championships, in advance of the game's release, four championship players were made available to participants. Those are available in the retail version of Super Mario Maker, as well. To access them, play and complete 10 Mario Challenge courses to unlock courses within your Coursebot. Once you unlock everything but the last row, you'll gain access to the 4 championship courses.
Mario and Thwomp Death Sound Effects
If Mario and Thwomp repeatedly fall into the same bottomless pit at nearly the same time (it seems to work best when Thwomp just barely takes the dive first), a series of weird sound effects follows and lasts around 27 seconds. The sound effects won't play in Edit mode.
If you clear the final level in 10 Mario Challenge or 100 Mario Challenge, you'll meet up with Toad or the Princess. If you are dressed in certain costumes, you'll get individualized responses. Toad stresses out if you rescue him as Bowser, but Peach is merely bemused. Wear the Toad costume while rescuing Toad and you'll be greeted as "Captain."
Fly Swatter Mini-game
In the old Super Nintendo classic, Mario Paint, it was possible to play a Fly Swatter mini-game. You can play it in Super Mario Maker, as well. As you create levels, you will eventually see a group of three flies buzzing around on the screen. If you tap them all on the gamepad, you'll initiate the Fly Swatter mini-game. It has changed a bit, but should still be familiar.
Fun on the Title Screen
Once the game's title appears with a sample stage background, you can interact with various parts of the screen to produce interesting effects, or tap a general spot on the screen to summon a random enemy.
Tap the "Super" part of the title and a crowd shouts out the title while Super Stars appear
Tap the "M" in "Mario" to change it to a "W" and get thanks from Wario
Tap the "A" in "Mario" to trigger a shower of stars
Tap the "R" in "Mario" to stretch it and reveal a bunch of coins
Tap the "I" in "Mario" to summon a parrot, used in the Sound Effects editor
Tap the "O" in "Mario" to summon a Koopa Clown Car
Tap the "M" in "Maker" to drop a Buzzy Beetle shell
Tap the "A" in "Maker" to start a countdown before the letter blasts into space
Tap the "K" in "Maker" to give the screen a new/old look and music
Tap the "E" in "Maker" to drop a trampoline
Tap the "R" in "Maker" to stretch out the letter and produce a Kuribo's Shoe
All Shook Up
When you grab and shake objects as you are building your course, they will respond in different ways, depending on the object. For example, the Goombas will shrink and fly off the screen, and the Koopa enemy will turn colors. Let go of the wings and they will fly away.