When Rayman first enters the Heart of the World, go left until the player sees a column with a ledge peaking out, as there is no other column like this in the room. Climb up and go left on the 2nd floor. Then step on the switch, and go into the first secret room, full of all sorts of gems and jewels, including a green one at the bottom!
In the third area, across the wooden bridge is another first in the game: A Matuvu. Looking at it will earn the player 250 points.