India is a large country, you can't see everything in one go. If you are traveling before the trip, please do your best to not fall sick. Since we are in South India, my recommendations are in the geographical surroundings
R’s recommendations
Tamil Nadu (where Chennai is located) is famous for its Temples and architecture. Cities to visit - Chennai, Madurai and Thanjavur. You can also visit the Western Ghats landscape either Ooty or Kodaikanal (do not do this if you are prone to motion sickness).
SriLanka - It is a beautiful country, I have family there. Tropical paradise. Cities to visit - Colombo, Kandy, Nuwareliya, Mirissa. There are many national parks and resorts for you to choose from.
Kerala - neighboring state to Tamil Nadu. Beautiful backwaters and layered landscape.Cities to visit - Cochin, Munnar, Backwaters of Alleppy, Wayanad. If you are going to Kerala, definitely take a boat house stay in the backwaters
Goa - the most touristy of places in South India. Classic beach vacation. Former Portuguese territory.
Andaman & Nicobar - Island strip away from the main country. Surfing, deep sea diving, etc. There are still cannibal tribes present so best to follow suggestions from Lonely Planet or find a guide
Pondicherry & Auroville - Former French territory near Chennai is perfect for a couple of days or a staycation. Highlights are French Quarte, Tamil Quarter, Beaches and Auroville
Karnataka - Heritage sites and wildlife sanctuaries. Cities to visit - Bangalore, Mysore, Hampi, Gokarna, Mangalore
Chennai & Mahabalipuram - The wedding venue is closer to Mahablipuram. We are planning a small trip to see the monuments in Mahabalipuram and the Elliots beach in Chennai on the 4th for anyone who would like to visit. Will send more details when I have a concrete plan
Beach culture in India and Sri Lanka is different from the West. Commercial beaches like Marina(Chennai), Elliots(Chennai, Galleface (Colombo) will not let you sunbathe or swim. Bikinis will garner weird looks. These public beaches are more for walks, street food, and fairs. Private beaches, Resort beaches, and beaches in Goa can be used for swimming and sunbathing.