Finish Rapids Race (in the Rapids Ride mini-game) to get the corresponding prize within the indicated time. Note: You cannot get multiple prizes at once. Thus, you will have to do this multiple times and pace yourself carefully to get all prizes.
Chamber Stone, 100 Rupees: Finish in under 30 seconds.
Heart Piece: Finish in under 40 seconds.
Secret Seashell: Finish in under 35 seconds.