I love the mini-battles!  This cheat lets you play all of them.

Mini-Battles - Yoshi’s Island: Super Mario Advance 3 - Gameboy Advance

Here’s the cheat:

•  Bonus Stage Menu
• At the Stage Select Screen
• Hold the Select Button (don’t let it go)
• At the same time press:
  • L
  • L
  • B
  • A
  • R
• A menu appears and you can play all of the mini-battles!