Since you have only a single day in Gangtok, I suggest starting your day early at around 8:00 AM after reaching there. Here is a suggested itinerary for your day trip to Gangtok on 25th December:
8:00 AM - Arrive in Gangtok
8:30 AM - Have breakfast at a local cafe cafe restaurant
9:30 AM - Visit Encheyeyastery to experience the spiritual and serene ambiance
11:00 AM - Head to Ganesh Tok for a panoramic view of the city and surrounding hills
12:30 PM - Have lunch at a local local offering authentic Sikkimese cuisine
1:30 PM - Visit Namgyal Institute of Tibetology to learn about Tibetan culture and history
3:00 PM - Explore the handicrafts and souvenirs at the Directorate of Handicrafts & Handloom
4:30 PM - Visit MG Marg for some leisurely shopping and strolling
6:30 PM - Enjoy the scenic beauty of Gangtok during sunset at Tashi View Point
8:00 PM - Have dinner at a local restaurant or food joint
9:30 PM - Pack your bags and head to the hotel for a restful night's sleep
Please note that the actual timing may vary based on your interests, availability, and travel time between the attractions. Additionally, it's advisable to book accommodations in advance and keep a buffer in the schedule to account for any unexpected delays or changes. Have a great trip!
North sikim & nathula