What are can't miss places to visit in India?

India is too big and diverse to answer that easily. I liked Manali, and the Gompas north in Himachal Pradesh. But beware of loud, stoned Israeli tourists, unless that's your thing.

i really enjoyed amritsar in punjab ... both the golden temple and the waga border were a blast.. i also really enjoy dharamsala and spending time/taking a few classes at the library there.

also really depends on the time of year you're traveling.. and what you'd like to see.. Check out Lonely Planet India

The Golden Temple is the best tourist attraction in India, bar none. I would put that at the top of my list. If you go to the north I suggest climbing up Vaishno Devi, which is a big Hindu pilgrimage. Anywhere in Rajasthan in the west will be beautiful. If you go south then hit up the tea gardens in Munnar. Also almost anywhere in Kerala will be nice. Finally, the Arunachaleshwara Temple in Tamilnadu is really cool and the oldest temple in south India.

India is a popular tourist destination in the world. It is frequently visited by global tourists. It is known for its enchanting and charming destinations. Nature has bestowed all its forms to India. Kasauli, enriched with mixed forests of pine, oak and huge horse chestnut is a popular hill station situated in Himachal Pradesh. Historic Victorian mansions, sky kissed mountain, dense forests and lush foliage are some among the prime attractions of Kasauli hill station. Fabulous flora and fauna present in Kasauli tourist places makes the land an orchard of nature. These provide relaxation and a more personal atmosphere that is for what people come from far areas to enjoy in this hill station.
http://focuztours.com/destination/kasauli-tourist-places.html