To cheer for your opponent, press any the D-Pad in any direction.
To give your player different clothes, hold a C button when choosing them.
Select 4-player mode, and have all players choose either Yoshi or Donkey Kong. Then go to any course, except the mini-golf courses, and a hidden tune will play.
Hold Z and R together, then SELECT the clubhouse option with A and the code entry option appears in the next menu.
When playing for a new character or anything, to re-do a hole if you mess up save and quit and then load that game again. It will start the hole over again.