Hold R1 + L1 + Square and press Start at the opening screen.
Relic Round
When you get to "Warp Room 4", go back to any level you have already visited to find a relic (after you press X to chose that level). You will then play a harder version of that level. To actually defeat this level, you must win two times in a row.
Platinum relic race
After you defeat the last Boss you can go to the 5 War Room. If you win one of the cups, the Gold Relic race and a Platinum Relic race will be unlocked. Defeat the champion of this course three times. You can jump back to other worlds and do the Platinum Race after you have the Gold Relic.
Infinite points in Mallet Mash
Pause the game and press [Up] , [Down] ,[Square] , [Triangle] and then unpause the game.