In level 1 at the end of the level, DO NOT pop the big balloon! On the right-hand wall of the balloon, there is a secret block. Jump UP to hit it. After you have discovered it jump LEFT. There is a secret passage. When you cannot go any further press UP.
Here are some passwords to help you out.
Level 8 -- Balloon, 2-Up, Star, Moon,1-Up
Level 15 -- 2-Up, Star, Moon, 1-Up,Balloon