I'm not sure where you are travelling from but I suggest you if you are travelling from Bangdogra do go to Kalimpong on the way to Gangtok and return stopping at darjelling. Sikkim is a small state you must plan with group and book taxi for North sikkim covering Gurudongmar Lake and yumthang Valley. You can book directly once you reach Gangtok there are various agents and If in group it will cost you less. Enjoy! Maybe someone else can reply about Package. But I suggest you to book yourself and plan it accordingly.