India is so large and varied that any list of highlights depends entirely on what kind of traveler you are. These are the things that reward the effort most reliably.
The Golden Triangle — Delhi, Agra, and Jaipur form the classic first-timer's circuit and there's good reason it's popular. The Taj Mahal, the chaos of Old Delhi, and Jaipur's painted bazaars are all extraordinary, and the circuit is well-served by rail.
Rajasthan — Rajasthan deserves more than a quick hit on the Golden Triangle. The desert forts at Jaisalmer and Jodhpur, the lake city of Udaipur, and the camel market at Pushkar each merit several days.
Varanasi — Uniquely confronting and uniquely alive. The cremation ghats, the riverside rituals, the labyrinthine old city, and the Buddhist site of Sarnath just outside town — no other city in India concentrates this much intensity in one place.
Kerala's Backwaters — Kerala is the most relaxed part of India. The network of lagoons and canals behind the coast, explored by houseboat, is genuinely beautiful and low-stress by Indian standards.
Hampi, Karnataka — The ruined capital of the Vijayanagara empire, scattered across a boulder-strewn landscape in Karnataka. One of the most atmospheric historical sites in all of Asia, and still undervisited.
Himalayas — The mountains of Himachal Pradesh are a full destination in their own right — trekking, monasteries, the Dalai Lama's exile seat at Dharamsala/McLeod Ganj, and some of the most dramatic scenery on earth.
Kolkata — The former British imperial capital is India's most literary and politically intense city. Great Victorian architecture, a world-class art museum, the best Bengali food in the country, and a completely different vibe from the cities of the north or south.
Goa — Goa is the default rest stop, and for good reason. The beaches are excellent, the former Portuguese colonial architecture in Old Goa is worth seeing, and the combination of cheap food, easy living, and warm water is genuinely restorative after hard travel elsewhere.