“Astronomy is the only subject taught to both Wizards and Muggles.”
The Haunted Observatory is an annual gathering of long-deceased scientists, fictitious brainiacs, and other human(oid)s. Professors Snape and Sinistra are both common occurrences, along with the likes of Isaac Newton and Carl Sagan. The kids who come are treated to a scavenger hunt dedicated to finding all of the famous (ghostly) folk roaming around. The parents who tag along have a chance to deepen their well stellar knowledge, or simply walk and gaze at the many fascinating astronomical artifacts. Children are free to frolic, and the adults can come at zero cost.