If you are planning to visit Jaipur aka Pink City and wanna get your mind blown, Jantar Mantar should be on the top of your list.
You will find so many forts and palaces in Rajasthan but you won't find another place like Jantar Mantar.
It is the perfect example of the amazing application of mathematics, Science, Engineering and Architecture. The place is full of instruments which used to be used for predicting the time and future happenings. Which is something very different from other places for sure.
Infact I suggest you that if you really wanna know everything clearly about Jantar Mantar, hire a Guide. Otherwise it will be just another old place for you and you won't really enjoy the visit.
Jaipur is an amazing city to visit and Jantar Mantar is just a tiny part of it. It takes 3-4 days to really visit everything. So don't come for 1 day and expect visit everything within hours.
You are welcomed anytime.: -)