The Kodaikanal Solar Observatory, founded in 1899 by the Indian Meteorological Department, is one of Asia's oldest and most significant astronomical research stations. Perched above the town on a ridge with an unobstructed view of the sky, it has spent over a century recording the sun's activity in meticulous detail.
Visits are possible on weekday mornings with prior arrangement. A guide from the observatory takes small groups through the dome, explains the equipment (some of it original and still in use), and discusses the solar cycle research. It is not a tourist attraction in the standard sense — it is an active research facility, and the visit feels accordingly serious and interesting. If you have any interest in science or astronomy, this is the best thing to do in Kodaikanal.