Pokhara is the starting and ending point for many of the popular trekking routes in Nepal. Longer treks (one to three week long) such as the Jomsom trek, Annapurna Circuit, and Annapurna Sanctuary begin here. Check a book on Nepal trekking for more details on this. Otherwise, you can also visit this site Pilgrims Book House for information on trekking books of Nepal.
For those with less time, Pokhara also provides shorter (one to seven days) more leisurely treks around the neighboring hills and villages. The popular ones are: