The Kathmandu Valley, which is actually home to four different ancient cities, presents such an array of sightseeing that it is unlikely that a single trip to Nepal will be enough to see all that is on offer. The ancient capitals are crowded with beautifully crafted buildings dating back thousands of years. In particular, the old palace squares of Kantipur (Kathmandu), Bhaktapur and Patan have all been faithfully restored in recent times. A glimpse of some untouched ancient monuments can be seen at the hilltop citadel of Kirtipur, a short drive to the west of the main city.
It was once said that Kathmandu had as many temples as houses and the plethora of shrines, both Hindu and Buddhist, around the city would seem to bear this out. Of particular interest are the Hindu shrine at Pashupatinath, and the two massive stupas at Bouddhanath and Swoyambunath (the monkey temple)
Add to this short day trips to the valley rim for views of the mountains and you could easily spend days in the valley and still not see all that is on offer.