These are the must-visit UNESCO World Heritage Sites in Nepal:
Cultural Heritage Sites
Kathmandu Valley (including Swayambhunath, Boudhanath, Pashupatinath, Changu Narayan, and the three Durbar Squares: Kathmandu, Patan, and Bhaktapur)
Lumbini: The Birthplace of Lord Buddha
Natural Heritage Sites
Sagarmatha National Park
Chitwan National Park
You can visit Historical Monuments in Nepal: Entry Fees and Opening Hours to know more about the UNESCO World Heritage Sites in Nepal.