Sorry you didn't get trekking infos here, as it looks like many of us have
no experience about it.
I don't know if by trekking you mean climbing mountains or simply hiking
around. I've been in Kathmandu valley many years ago and was happy to learn
you can really walk all the valley around, up and down the hills, instead
of taking bus and taxi rides. Every spot is never too far for a good walker
and, if tired, you can always switch to public transportation.
The town of Pokkara, facing the Himalaya range, is the departure place of
many easy trails at the foot of the mountains; in any case I'd suggest
visiting one of the many trekking agencies in Kathmandu, as you'll probably
need assistance for food and accomodation while in the country. Some places
are still rather savage in spite of the tourism.
Have a nice planning