Red Coins in the Sand Castle
From the Super Mario Wiki
Returning to the secret area will allow the player to use F.L.U.D.D. and do a Red Coin challenge. On the second green platform is the Red Coin Switch, and Ground Pounding it will cause eight Red Coins to appear on the path ahead and on the sand castle. The player has a time limit of one minute to collect them all; failing to do so will result in losing a life. Three of the Red Coins are along the Sand Block path, with the next five scattered around the sand castle. Collecting all eight Red Coins will have the Shine Sprite appear at the base of the sand castle.