What does Bruno do in his spare time in The Boy in the Striped Pajamas?

John Boyne's The Boy in the Striped Pyjamas tells the story of nine-year-old Bruno; his twelve-year-old sister, Gretel; and their parents, who live in World War II-era Berlin. Bruno's father is a Commandant in the Nazi military and is tasked with overseeing Auschwitz (which Bruno naively calls "Out-With").
Bruno is unhappy with the move from Berlin to Auschwitz, specifically with relocating far from his friends. Gretel becomes friendly with some of the soldiers at the camp, and Bruno is left to his own devices. He begins to explore the fence around the camp after catching sight of some prisoners from a window in his home. During his exploration, he meets a young prisoner named Shmuel. Bruno's perspective on Shmuel's life is that he is allowed to play outside all day in his pajamas.
Bruno excitedly decides to sneak into the camp to help Shmuel find his father. Shmuel brings Bruno an extra set of the striped clothes so that he can sneak in undetected. A bout of lice already caused Bruno's mother to shave his head, and he fits in seamlessly. Unbeknownst to Bruno's parents, he is in the camp when his father instructs his men to exterminate some of the prisoners, including children. Bruno and Shmuel are among the prisoners led into the gas chambers, and the close of Bruno's story becomes clear.