KID NIKI

On Level 3, get on the third rising and falling cloud. Then
kill the kite and the bird, and ride the cloud all the way
up. Press up arrow while jumping, and you'll find a secret
room.

Free man:
Every time you kill a boss, a scroll appears. You can magically
change that scroll into a free man. Wait until the first and 
last digits of your time are the same then hit the scroll. This
works every time for every boss!