Well, he might encounter rare and dangerous beasts. Maybe he discovers a hidden power source or an ancient artifact. He could also meet some reclusive and powerful shinobi who have made the forest their home.
Naruto in the Forest of Death in fanfiction might form alliances with the native creatures. He could befriend a giant snake or a group of intelligent monkeys. These new allies could help him in his future battles or in uncovering the secrets of the forest. Also, he may find a secret training ground within the forest that was once used by a great ninja in the past, and use it to enhance his skills.