The Jantar Mantar are astronomical clocks / calendars. Five of them were built in early 18th century India by order of Maharaja Jai Singh II. The others are in Jaipur, Varanasi, Ujjain, and Mathura. The one at Jaipur is said to be the most impressive, and I can attest that it’s more impressive than the complex in Delhi. However, I’ve only visited the two. What we read of the Varanasi one didn’t make it sound worth the trip, even though we were in the area of it. (It’s atop one of the buildings near the main ghat.) I haven’t been to the other two cities, but I’ve heard that not all of the Jantar Mantar remain intact, so they may just be ruins.
When we were visiting the location in Jaipur, a guide asked: “Do you believe in these things, astrology and astronomy?”
To which the natural response is: “That’s like asking whether I believe in ghosts and gravity.”
At any rate, if you are into science these sites are worth your time.