I have been to Nepal 5 times already (cos I used to be a tour leader specializing in Nepal and India).
If u are just going for sightseeing, maybe just a little bit of leisure trekking, then I would advise that the best time to visit is between October to November as this is the best time to view the beautiful sceneries of the country. Although it can be quite cold and chilly during the evening/night, but it is still better than going to that country and not seeing any of the beautiful mountains like the fishtail mountain and the whole range of Himalayas. Just bring enough heat-packs, thermal clothings and a windbreaker and u should be comfortable.
During clear days, the mountains have different views during different time of the day. At sunrise and sunset, the Himalayas is orange in colour... Simply spectacular!
If u visit during a time (during the monsoon season) when the sky is not clear, u will not be able to capture any beautiful scenic views at all.
The monsoon season starts about mid July till maybe late september, lots of rain and it will be cloudy. There was once when i was there in July, the monsoon washed out most of the highways and road.
So the last time, in order for us to travel to Pokhara (one of the tourist attraction where its best to see the fishtail mountain and Himalayas), we had to take the domestic flights which are mostly forker planes. Moreover, the runway is pebble ground... so can u imagine the landing??
Best of all in Pokhara during clear and cool days, u can either rent bicycles and cycle along the river or go on a boating trip to enjoy the reflection of the mountains on the water. Its beautiful, trust me.
Have an enjoyable trip... Cheers!!!