Virtual Boy system and controller
Locate the room in level one built from girders with enemies that appear from left to right on the floor. Stop and shoot the wall on the left side at the end of the room, just before the exit to the next room to reveal a Virtual Boy. Shoot the wall directly the Virtual Boy to reveal a controller.

Mini-house
Locate the room in level one with small figures running about. Eliminate all enemies, stop, and turn around to face the entrance of this room. Slowly move along the wall on the right side and shoot the bottom right corner of the wall near the door.

Kangaroos
Destroy the first three enemies in the first room on level two, then turn around.

Bikini girl
Leave the first room on level two and continue moving forward until a small hole guarded by a cat appears. Enter this opening and shoot the lower right wall.