I lived there for almost three years. It was one my best 3 years of life in Kathmandu, Nepal. The city consists of 7 world heritage sites namely
Pashupatinath, main reason for Hindu to go there
Swayambhunath and
Boudhanath, main reason for Buddhist to go there
Kathmandu darbar square
Bhaktapur darbar Square
Patan darbar Square
Changunarayan temple
These world heritage sites and many more tourists attractions like nama boudha, hiking option, taudaha pond, white gumba, white House and many more. I almost been to every single place and it was an awesome experience to witness the history of the valley the art and architect, nature and environment and hospitality everything there was great.
The only is that the valley is little polluted because of the population there maybe. But in overall aspect you will have a great time like I had when I was there.
If you want to go they make a trip of a week or more less than that will make you miss many thing. Trip will be very budgeted because its cheap there to go and buys things or eat. Now how to reach there. For Indian it's very easy you can reach by bus which you can get from Delhi costing exactly 2300 and you can take the flight costing around 5000 to 6000 depending on the time of booking and NO VISA is required.
Go grab your ticket and pack your back because this winter its Kathmandu time.