Possibly transferring over to Ktm for work, and company have funded a visit over there, for two weeks, including DC (who are 14, 13 and 4, near 5). We will be staying with the coordinator in the neighbourhood we'd most likely live in, called Lazimpat, in the NW part, fairly close to the Palace Museum, I believe but residential and non touristy. A lot of that time is spent doing practical, not touristy stuff, to do with the schools and looking at moving there, seeing estate agent, I'm also going out of Ktm for two days to see one place where I'd be working at.
However, that leaves a good few days to do touristy stuff 
DC have been on holiday twice, and the furthest abroad we've gone was four hours by plane, so looking for some fun!
So far have planned on visiting the Palace Museum, the Boudhanath stupa, Durbar square and Chitwan national park, six hours away. We can go to further away places etc; as accommodation, flights, most meals etc; have all been paid for.
Where would you go? DH and I can split up, if there's something which wouldn't suit both teens and a younger child.