The Secret of Lost Creek (1989)

Siblings Jennie and Robert Fogel are spending the summer in the small town of Lost Creek in the Sierras. Both are sure they'll be bored, but when Jennie stumbles across clues leading to a lost treasure and Robert becomes convinced he has spotted Bigfoot, their vacation takes on new excitement. Investigating the past of the town and its environs and geography, they find clues to locate the lost treasure, turning bored days into a great adventure.