Hi there!
I travelled to Sikkim in the year 2018 for 5 days. In this post, I have chalked out my itinerary that I followed for Sikkim. Read this post for 7-8 days itinerary or 3-4 days itinerary.
Day-1: Do a city tour of Gangtok covering Ganesh tok, Tashi viewpoint, Lhasa waterfalls, Himalayan zoological park, Lingdum monastery, Gonjang monastery, Banjhakri waterfalls, Bakthang waterfalls.
Read this post for Gantok's itinerary.
Day-2: Take a day tour of Nathu La mountain pass which will cover Baba Harbhajan Singh temple and Tsomgo lake.
Read this post for Nathu La's itineray.
Day-3: Leave for South Sikkim. Visit Char Dham temple, Samrduptse hill, Buddha park and stay in Pelling.
Read this post for complete South Sikkim itinerary.
Day-4: Go for Pelling sightseeing; covering Rimbi waterfalls, Orange garden, Kanchenjunga falls, Khecheopalri lake, Pemayangtse monastery, Rabdentse ruins.
Read this post for complete Pelling itinerary.
Day-5: Return