Sikkim

Sikkim is small, almost entirely vertical, and organised around one mountain. Kangchenjunga is worshipped here rather than climbed, and Khangchendzonga National Park was inscribed in 2016 as India's first mixed World Heritage Site, listed for its culture as much as its glaciers. A realistic trip is Gangtok, then North Sikkim to Lachen and Lachung, then west to Pelling and Yuksom. That is a full week, because the roads are slow and every day has a mountain pass in it. The usual mistake is adding a fourth region and spending two of those days in a car. Permits shape the whole trip. Foreign nationals need one simply to enter Sikkim, and Nathu La and Gurudongmar Lake are closed to them outright; Indian travellers need a Protected Area Permit for both, arranged through a registered operator rather than on the day. Gurudongmar sits high enough that altitude, not weather, usually decides how it goes. October to early December gives the clearest mountains. March to May is the rhododendron season. June to September is monsoon, landslides and closed roads.