The Museum of Mental Health is on the grounds of the Oregon State Hospital and explains the history of the institution, mental health treatment, and daily life inside the hospital. It also covers the local connection to One Flew Over the Cuckoo's Nest, which was filmed with help from hospital staff and patients.
The subject matter is serious, but the museum adds a distinct piece of Salem history that most visitors would otherwise miss. Hours can be limited, so check before making a dedicated trip.