Beta Treasure Chests: That's right, you heard me! Beta treasure chests from an old version of Zelda: Ocarina of Time were put into this game! Take a look and see for yourself!
Beta Mini-Dungeon: Another surprise! The Skulltula House (not sure which, though most likely the one on the marshland) is actually a remake of one of the beta dungeons from Zelda 64! So remember that next time you're in that Skulltula House, you're actually in what was once a beta dungeon!
Beta Quest: Yes, the Beta Quest is in Majora's Mask! If you use the Fight Majora's Mask Demo code and die (or win), you'll be brought to the swamp with nothing except a sword. All the exits lead back to the swamp. As far as I know, you can't escape it, so it's still not technically the Beta Quest... yet. We're much closer, though!