In spring (March to May) it is more quiet then in autumn. The advantage of going on a trek in spring is that there are less people on the trails then in autumn. An other advantage is that the days are a lot longer. It is mildly warm at lower altitude and quite moderate temperature above 4.000m/13,123ft. And you get to enjoy the beautiful blossoming rhododendrons and other beautiful wild flora.
In summer (June to September) it`s monsoon in Nepal: heavy rainfall makes most of the trekkings in this season unpleasant. Nonetheless is this the perfect time to do a trekking in some regions of Nepal connecting to Tibet like Mustang and some desert parts of Nepal like Nar-Phu valley and Dolpo region. In these areas there is not a lot of rain during monsoon because they are situated in the `rain shadow` of the Himalaya. Higher trails are also perfect possible, since you walk then above the clouds.
In autumn (from half of September until half of December) the weather is nice and the temperature is moderate. The advantage of going in this season is a bigger chance of blue sky with endless panoramic views. Occasionally short storms may dump considerable snow at high altitudes.
This is the most popular trekking season in Nepal.
In Winter (half of December until March) the days are generally clear. There is snow fall in the higher mountains with strong winds and cold temperatures. Though, winter is a great time to do a short trek in the Kathmandu Valley or a trekking in the lower mountain areas.
You can fill in your remaining days in Nepal with a wide range of activities. Discover some days the jungle in one of the nationals parks in the South of Nepal, go enjoying the shores of Phewa lake in Pokhara or visit the cultural sites in Kathmandu Valley.