If are interested in outdoorsy stuff like hiking and rafting, and seeing architecture, temples and things like that, you won't have to venture out for these. You can visit Old Kathmandu, Boudhanath, Pashupatinath and Patan (Lalitpur), which are within a couple of hours from Kathmandu. Kathmandu Valley has a few places of interest like Dakshinkali temple so it's also worth checking out.
I would also add Bhaktapur and Changu Narayan in the Kathmandu Valley. Bhaktapur is about 8 miles east of Kathmandu and has lovely cobblestone streets and an stunning golden gate. You can get there easily by bus or taxi from the capital. Changu Narayan is a village not far from Bhaktapur and worth the visit because of the temple there surrounded by ancient stone sculptures.
Kathmandu short treks There are many short treks you can do near Kathmandu ranging from day hikes to short treks of 2 days, 3 days or even longer. Champadevi is the easy hikes near Kathmandu and offers stunning sights of Buddhist monasteries and beautiful views of Kathmandu valley and the Himalayan Panorama.