Jantar Mantar in Jaipur
Jai Singh II built Jatar Mantar in 1726 and it remains the best preserved of the five observatories he built within his lifetime. Armed with knowledge of European astronomical advances, Jai Singh II invented many of the instruments himself, the most impressive of which is the massive Samrat Yantra sundial with a shadow that moves up to 13 feet (4 meters) per hour, or a hands-width every minute.
Guides onsite can explain how the devices work, and several are still used to make astrological and weather predictions. Try to come on a sunny day, since none of the devices will work under cloud cover, but avoid the middle of the day when the sun is particularly hot and shade is harder to come by.