Mystery of the Secret Room

From the back cover: When your great-aunt Celia decides to take off on an expedition, she leaves you in charge of her strange, old house. It looks as though she won't return by the end of the month, so you follow the instructions in her note. You open the door to a secret room where you find three boxes - and another note from your aunt. "Open one - but only one - of the boxes!" it says. Which box will you choose?