The Boudhanath Stupa with 36 meters high with dome in the east of Kathmandu is the biggest stupa of Nepal. Built in the 5th century it is also one of the oldest stupas of Nepal. 
The Boudhanath Stupa is surrounded with more than 50 tibetan monastries and is a UNESCO world heritage site since 1979. In the outer ring of the stupa people circumbulate and you can find here small shops, restaurants selling Momos and  Thupka, a few monestries. Make sure you dine on one of the rooftops and see the sun set.  Every full moon there are celebrations at Boudhanath Stupa.