While others have mentioned Rajasthan, Kerala, Pondicherry, Goa, I think only one poster mentioned Khajuraho. If you do get the time, try visiting some of the temples in Southern India. The architecture you will see in Northern India/Rajasthan - dates to between 1400 and 1900 - the Mughal/British Raj period. There is a layer of India that goes earlier than that.

A few examples include Hampi (a UNESCO World Heritage site), Belur/Halebid near Bangalore in Southern India. There are tons of places in Tamilnadu, as well. The Thanjavur/Tanjore/Chidambaram area has many classic examples from about the same era (or a bit later) than the Angkor Wat temples.

What strikes me is that some of these temples are probably the longest continuously used places of worship. If History interest you, pick one temple to visit. I'd recommend either Hampi or Tanjore's Brihadeeswarar temple.