It is hard to say where to go , all area are interesting and have it's own uniqueness. Here are the major tourist attarctions
Golden Triangle: ( Delhi - Jaipur - Agra Taj Mahal ) - the most puplar tour , can combine with Varanasi , Khajuraho
Rajasthan: Jaipur , Udaipur , Jodhapur, Puskar, Jasemelar
Buddhis circuit - Ajanta Elora caves, Boudhagaya, Sarnath, Kushinagar , ( Lumbini - Nepal)
Kashmir: Srinagar area and Ladakh
Tamil Nadu: Ooty and Kodaikana
Goa, Karnataka area
Orissa : Bhubaneswar | Sun Temple
Gujarat: Dwarkadhish Temple
Sikkim: Gangtok, Darjeeling
Maharashtra: Juhu Beach | Nagpur
Madhya Pradesh: Bhimbetka Caves | Bandhavgarh
Himachal Pradesh - Gharawhal Himalaya , Badrinath, Kedarnath, Gangotri, Yamonotri
Yoga and rafting : Rishikesh, Haridwar