A special feature allows select Amiibos to unlock an extra planetoid in the main hub based on the character used. These special planets contain 1-Up Mushrooms (1 each).

Amiibo Planet & Feature
Mario A small platform with a castle based off of the original Super Mario Bros. complete with raising and lowering flag. You can also slide down the flagpole.
Yoshi The wooden Yoshi head from Mario Galaxy 1.
Donkey Kong The Classic set of Girders from 25m, with barrels rolling down the girders.
Link Link's house from Skyward Sword.
Zelda The Sacred Realm from Ocarina of Time, with illuminating sage symbols.
Samus The Gunship from Metroid Prime, which can actually fly around the hub.
Kirby The Fountain of Dreams from the series, which can spew forth starbits on occasion.
Fox The Great Fox from the series. Rarely, it fires at asteroids, which brings forth a shower of Starbits.
Pikachu Viridian Forest from Pokémon Red, Blue and Yellow. The 1-up, along with the sling star exit, is at the end of the maze.
Diddy Kong Donkey Kong's treehouse from Donkey Kong 64, with a Golden Banana inside that functions as a jukebox.
Luigi The original stage from the original Mario Bros, with Koopa Troopas and Crabbers standing in for the classic Shellcreepers and Sidesteppers.
Captain Falcon The classic track Mute City from F-Zero X, complete with Dash Peppers for practice racing.
Little Mac The complete World Circuit boxing ring from Punch-Out!!; some "king"-based bosses can be fought in a rematch here.
Rosalina The Gateway planet from Galaxy 1 and 2. Not very useful, but decorative.
Bowser A minaturized Bowser's Castle from Paper Mario.
Lucario A recreation of Pokémon Diamond and Pearl's Spear Pillar, complete with the Hall of Origin; you can even face off with Dialga, Palkia and Arceus (but only one per hub world; changes at random each time its visited).
Mega Man 
Ness Magicant from Earthbound.
Pac-Man A 2D platforming version of the Pac-Maze from the classic Pac-Man
Greninja Prism Tower from Pokemon X and Y.
Jigglypuff A recreation of Mt. Moon, complete with Parasect Goomba reskins.
Palutena Palutena's Temple from Kid Icarus uprising.
Dark Pit
Zero Suit Samus
Ganondorf Ganon's Castle from Ocarina of Time. The inside is explorable and everything.
Dr. Mario A pseudo-3d version of the classic NES Dr. Mario.
Bowser Jr. The Koopaling Airships from New Super Mario Bros. U. They have Sling Stars to help travel from ship to ship.
Mr. Game & Watch