In-game reset
Press Start + Select + A + B during game play to restart.

Secret room
Get the Power Bracelet that allows you to kick blocks. At the main screen where you can choose to play a new game or continue, kick one of the blocks to reveal a passage to a secret room. Save two keys from the fifth geyser, and use one from the stone circle under Gruk's house. When you complete the game, go to the secret room, and you can open the door. A man will open the middle door on the new game/continue screen, which lets you start a new game with all your previous stats and items.