The Outdoor Girls at Rainbow Lake is a novel by Laura Lee Hope, first published in 1916. The story follows a group of friends, Grace, Mollie, Betty, and Amy, who call themselves the Outdoor Girls, as they embark on a summer adventure on Rainbow Lake. The girls are excited to spend the summer at a camp, but when they arrive, they discover that the camp is closed for repairs. Undeterred, they decide to rent a motorboat and spend their summer cruising on Rainbow Lake. They have many adventures and encounter new friends, including a group of boys who join them on their adventures. As they navigate the lake, the girls stumble upon a mysterious island that seems to be abandoned. They discover clues that suggest someone has been living there and try to unravel the mystery. In the end, they uncover a secret that leads to a surprising and satisfying conclusion. "The Outdoor Girls at Rainbow Lake" is a fun and exciting adventure story that will appeal to readers who enjoy tales of friendship, mystery, and outdoor exploration.