Monsoon season in Nepal is the month of June, July, and August. There is trekking region in Nepal which is a rain shadow area. Upper Mustang, Annapurna Circuit, Upper Dolpo are the major trekking destination during monsoon. Those areas are less vegetation similarly as Tibetan plateau. It receives very little rain in those areas; at the same time, other areas of Nepal get extremely heavy rain can be the whole day. Fewer vegetation areas get greenery after little rain, which makes the area more charming that is even beneficial for trekkers because greenery provides more oxygen which caused easy to breath at high altitude.
Annapurna Circuit Trekking in Monsoon season: the upper part of the Annapurna circuit like the Manang part is fewer vegetation areas get less rain. The starting part of the trekking gets heavy rain but still trekking at the area does not get block or disturbance. The regular trekking starting from Besisahar, reach rain shadow area of upper Manang, climb to Thorong La, and trekking ends at Lower Mustang at Muktinath Temple. Upper Mustang Trekking in Monsoon considers as main season. Plateau landscape gets very little rain caused valley became greenery. It becomes more charming with panoramic mountains. Similar to the Upper Dolpo Trekking is high Himalaya trekking also called rain shadow area. Similarly trekking to Limi valley at Humla, trekking to Rara lake is the best destination.
Many of other best trekking still possible to do during monsoon. Mountain views are often obscured cloud; temperature gets high, rains daily. You will get fewer crowds during that period. Annapurna region, Everest region, and Langtang region still will be possible to trek. This is the main season Nepalese farmers working in the field. You will view real-life of Nepalese peoples this season. Sightseeing tour in Kathmandu, Pokhara, and other major tourist cities do not disturb. Jungle safari in Chitawan, a white water rafting tour is also suitable at the period of monsoon in Nepal.