Best Time to visit Sikkim is October to December and March to May. Below is the itinerary we followed when we visited Sikkim:
Day-1:Flight to Bagdogra and reached Siliguri
Day-2:Siliguri to Gangtok and applied for necessary permits.Sight-seeing in Gangtok.
Day-3:Gangtok to Lachen.
Day-4:Gurudongmar Lake and Kaala Patthar. Reached Lachung by evening.
Day-5:Yumthang Valley and Zero Point.Back to Gangtok.
Day-6:Nathula Pass and Tsomgo Lake.
Day-7:Char Dham in Namchi. Reached Ravangla and visited Buddha Park.
Day-8:Rabdenste Ruins and back to Siliguri.
Gangtok- 2 days for local sightseeing as well as for Nathula pass n baba mandir.
Lachung, Katao(mt katao), Zero point and Yumthang valley- 1 Day
Lachen- 1 day for gurudogmar lake
Instead of Pelling, You should visit Yuksom- 1 day
Hi Shruti, 7 days is a very good plan for sikkim.
Day 1 & 2- u can cover Gangtok sightseeings and try paragliding too and visit Tsomgo lake or Nathula pass for second day.
Day 3 & 4- u can travel to South sikkim which has good parks,tea gardens and namchi.
Remaining days u can plan well for north sikkim which has adorable views like yumthang valley and lachen.
Shruti you can check this plan- The suggestive travel plan : Day1 Arrival at Bagdogra & transfer Gangtok (4hrs). Stroll around M.G.Marg, overnight stay Day2 Gangtok. - Tsomgo Lake, Baba Mandir & Nathula Pass. Day3 Gangtok to Lachung, Overnight stay at Lachung. Day4 Lachung - Yumthang - Gangtok. Day5 To Pelling via Ravangla (4.5hrs). Day6 Pelling - Darap Village, Rimbi Waterfalls, Rock Garden, Kunchenjungha Water Falls and Khecheopalri Lake. After lunch visit Pemayangtse Monastery, Helipad View Point, Rubduntse Palace Ruins and Bird Sanctuary Day7 Pelling To Darjeeling (4hrs), afternoon - roaming around Mall, coffee at Glenery's Day8 Darjeeling - Tiger Hill, Ghoom Monastery, Toy Train, HMI, Tea Garden Day9 Return journey. Hotels can be suggested if the budget is mentioned