While in the connection room to the Drablands, if you do a Pegasus dash (L1/R1) and the walls or pillars, a ball will fall from the ceiling. If you slash it, it will pop up in the air and you can keep hitting it to keep it in the air. Depending on how long you keep it up, you will get some bonus rupees.
Some of the costumes are unlocked by completing bonus challenges.
After clearing all 16 Challenges in a World (the normal run plus all 3 Challenges of each of the 4 Levels) go and talk to the King. He will give you a Gold Rank Material for that World which can otherwise only be obtained from the Colosseum. It doesn't matter if the Challenges were cleared in Multiplayer or Solo Mode, as long as all 16 Triangles are lit up on the Board. You can only receive 1 of each Material from the King.