The answer actually depends on your taste and how many days you have in your hand.
I would suggest dont miss out Jaipur and Udaipur in Rajasthan. It has beautiful palace, temple and fort. It is the best combination of past(heritage) and present(globalisation).
Kerala is famous for its clean beaches and backwaters.It it also popular for its alterate healing massages.
Goa is well known for its pristine beaches and laid back and relaxed lifestyle.
If you do visit India, Agra is a must for Taj Mahal (One of he seven wonders of the world).