OurGuest Travels (contact no. 9734813101) is the best tour company for North Sikkim tour packages. They have some very good homestays in North Sikkim including Lachen, Lachung, Mangan and Dzongu. They have good Innova vehicles with reliable drivers. They also make the entry permits for North Sikkim tours (which is needed since North Sikkim is a restricted/protected area). For the North Sikkim tour, you have to go with travel agents since you need official permits. So it is best to book with a travel company like OurGuest in advance (since the vehicle costs get very expensive during the last minute).