Scholastic Goosebumps Slappy World Attack of the Jack by R L Stine

Devin and his sister Violet are visiting their Uncle Jack for the summer. He lives in an old house by the seashore. Jack was a sailor and he has collected strange and fascinating items from the sea. Exploring a back room, Devin and Violet discover a locked trunk. A pirate's chest. The trunk is wrapped in old, heavy chains and locked with a huge rusted lock. But they manage to get it open.