We visited Gangtok, Darjeeling, Yumthang and Lachung during our 10 days trip. I will say, Lachung is one of the best places to stay. The river flowing through the town and traditional houses of the village gives you the most beautiful scenic experiences..In homestays Lachung , you can experience their tradition and culture while living with the locals. We also visited Nathulla pass, Changu Lake from there.
Many people have already mentioned many places to visit in Sikkim but I can see none suggesting to go to Gurudongmar lake though. Trust me, it's the most beautiful place in entire Sikkim situated at an altitude of 17,600 ft. If you want to see the real Sikkim then do pay a visit to that place. You won't regret.