Arthur Ransome's Swallows and Amazons (1930). A perfect book (a 12-book series, actually) to get readers into the summer spirit. The book chronicles the summer adventures of four siblings camping out on a little island, sailing around, confronting and forming alliances with a band of pirates (two other kids named Nancy and Peggy). It's a little bit like the Boxcar Children series...so much of it is very procedural: describing the provisions they bring to their island camp, learning new wilderness skills, cooking, swimming, and sailing. It's been a delightful read at the beginning of a delightful summer. (Picture above is a still from the 1974 film adaptation--it strikes me as a pre-Wes Anderson aesthetic).