Invincibility:
Collect the Spring Shoes and Towel, then go outside of the gym. Equip the Spring Shoes and Towel, enter a locker, and hold Up. Enter the "Equip" screen, and equip the Shoes and Bat. When game play is resumed, your character will remain in the locker, but may move freely. Although enemies may be attacked, your character will be invincible. Note: Falling will still be harmful.

Bonus room:
Collect the Spinal Zap and gain access to the elevator. Enter the elevator from the first floor and push "Up". Push "Down" when the indicator arrow is between "1" and "2". Repeat this until the indicator arrow remains between the first and second floors to enter room "00".