There are about three major treks in Nepal, including the Langtang region, Solu Khumbu region and Annapurna region. Out of these three Annapurna is considered to be the most accessible and boasts the widest variety of species. General advice for trekking in Nepal would be to stay out of monsoon season. However, it often happens that there is not as much rain as promised and plenty of sunny days. Either way, if you trek, try to consider the possibility of floods and rains during the monsoon season.