From Jalpaiguri railway station or bagdogra Airport, you need to book a cab till Gangtok,Sikkim. You can take a cab at the station/airport or you could contact a travel agent at Sikkim who will arrange pick up and drop for you.
Here is the itinerary without trek. Most of the trekking takes place from Pelling. You may have to skip few places below if trekking is on your agenda. On the way to Gangtok from Darjeeling, river rafting happens in Teesta river. You can check that out too.
*If you wish to enjoy a cable car ride, you might have to stay back for another day at Gangtok. You can then visit Rumtek monastery too.
Day 1 : Suppose you land at Jalpaiguri/Bagdogra by 11 am or 12 noon. It would be evening by the time you reach Gangtok. You can explore MG marg on this day. Stay overnight at Gangtok.
Day 2 : Visit Nathula Pass, Tsomgo [Changu] Lake and return to Gangtok. Stay overnight at Gangtok.
Day 3 : Start towards Lachen. Visit Tashi point on the way. The drive is very beautiful and you get to see many waterfalls. Stay overnight at Lachen
Day 4 : Start early morning [4.30 am] towards Gurudongmar Lake. You get to see Thangu and Chopta Valley on the way. Return to Lachen and further drive towards Lachung post lunch. Overnight stay at Lachung.
Day 5 : Drive early from Lachung towards Yumthang Valley [it’s a rhododendron sanctuary], visit hot spring there. Further, get to Zero Point. Drive back to Lachung. Have lunch and drive back to Gangtok.
* You may skip one of the above, if you want to save a day.
** Check local hotel/travel agents if trek is allowed in march in this circuits
Day 6 : Drive to Pelling from Gangtok. You can visit Rabdentse ruins. It takes minimum 2 hours over here as you need to walk for about 25 minutes till you get to the ruins.
Day 7 : You can opt for one of the following
Trek Hilley/Soreng- Barsey- Dentam-Pemayangtse
Trek Dentam-Tendong- Samdruptse – Namchi
Visit Singshore Bridge and Kanchenjunga Falls
Since climate around early march would not be the ideal time (mid-march / april is when the peak season starts) , best would be to check with local hotel/travel agents on availability
Day 7: Drive back to Jalpaiguri.
Few things to be noted :
1. Most of the places, need explicit visit permits. You can obtain them at Sikkim Tourism Centre at Gangtok (close to MG Marg)
2. Most of the Hotel guys (use tripadvisor for suitable accomodation) , would help in arranging permits/booking taxis
3. You might need a vehicle to travel to most of the places (only vehicles like sumo/maxx/xylo can operate in these terrains)
4. A random snowfall can disrupt some of your plans, account this while making your itinerary.
5. Calling and booking the hotels directly would be slightly cheaper (you can bargain both the hotel rates and travel around sikkim)
6. Cellular network is poor in parts of north Sikkim. BSNL works. If you want to stay connected, carry a spare bsnl sim card
7. Check if heaters are available at the place you stay or make sufficient arrangements
8. Food availability would be decent but nothing fancy.
Hope this helps ! Happy travelling and have fun!