Tsomgo Lake, also known as Changu Lake is a glacial lake in Sikkim and located at an altitude of 12,310 feet. The lake is filled with water of melting snow coming from the surrounding mountains. Most tourists combine visit of Tsomgo lake with Baba Mandir which is few kms away. A reserved large vehicle like Sumo costs about Rs. 4,000 for a round trip to Tsomgo and Baba Mandir, the shared vehicle rate is about Rs 450 per person. If you start by 8am, you should be back by 3pm. You can also additionally combine Nathula Pass (14,140 ft) and see the Indo Chinese border which would cost about Rs. 6,500 for a reserved vehicle and a shared vehicle around Rs. 900 - 1000 per person. However if one is just going for Nathula path to Sikkim just make sure to enquire about it before hand since it is closed for tourism sometime due to security and heavy snowfall.
You should reach Tsomgo lake before noon because in the afternoon the weather here usually starts getting bad. In fact best is to start from Gangtok by 7:30 to 8am and reach here by 10am so that you get clear 2 hours to enjoy. In the winter however the route sometimes get closed due to heavy snowing. You should spend some time here and soak in the serene beauty of the lake and its surroundings. In the earlier days the lamas used to study the color of water in the lake and forecast the future. A darker color would indicate that the coming year in the state may be troublesome. You can also take a ride on colorfully adorned yaks and ponies.
Note: If you have breathing problems, you are advised to consult a doctor before visiting these places (particularly Nathula) because of low oxygen at such high altitudes.