Are you trekking? or just seeing Kathmandu?
I trekked in the Khumbu in April/May some years ago. It was cold with snow up high, and wet with lots of rain on the hike out to Juri. It should be warmer in Kathmandu.
There's great character everywhere. The various Buddhist temples and the old parts of town are picturesque. There's good street photography everywhere. I recommend though that you get out of town and see some rural areas too. You have enough time to do some traveling there, even if it's just some bus rides out and around.