Go to the third pipe. Stand four blocks away from the pipe to the right and [Shift] + [Up] to jump to find a hidden box including a green mushroom that increases your life. Then go four blocks back and stand on the pipe.
Press [Down] to crouch. You will warp to asecret location. Stand under the pipe and jump to come back. Stand four blocks away from the pipe to the right.
Press [Shift] + [Up] to jump. Another hidden box will appear.
Part 3 (underground path):
Go to the third pipe and crouch. You will warp to a hidden location.
Keep going until you reach the end. Jump into the last pipe. As soonas you reach another location, go to the right. Jump on the dent on the wall and do a high jump. A hidden box will appear.
A red mushroom will come out of the box. It increases your stamina.
Keep going until you reach the top. Jump on the blue object shaped like a circle and press [Shift] + [Up] to destroy the walls. Keep going until you reach a pipe that has "3" written on it.
This will warp you to level 3. Continue until you see two pipes with Piranha Plants in front of each other in the opposite positions (one on the ceiling and one on the floor).
Stand on the first block after them and do a high jump.
A hidden box will appear.