Don't forget Nepal is an agricultural based society. This means locals go to bed early and get up with the sun. You will have to find your entertainment, after 9pm, in the touristic areas. Note that city laws in Kathmandu mean all music needs to be off by 10pm and people on the street should be on their way home.
On the Town
Casinos - 24 hour night-life does take place in the 4 Casinos of Nepal. You'll get free food and sometimes a disco if you put some money on the tables.
Culture Shows - Almost every restaurant and hotel that caters to the non-backpacking, up-scale market hosts a nightly culture show complete with local dances, music, songs and costumes.
Local Bands, the Bar Scene and Dancing - Look for local bands in the cities of Kathmandu and Pokhara that may even get away with staying open past the 10pm curfew. Discos are becoming popular and and dancing sometimes breaks out at local bars. Because of cultural attitudes toward men and women contact, local people general dance with the same sex. A general rule: if you see local women dressed in jeans, they've been cultured abroad or family ties extend outside the country.
Caution: with over 50 ethnic groups, drunken brawls are not uncommon whenever people are warmed up enough to cut-a-rug.
Movies - The theaters in the country play popular Hindi movies from Bollywood. Recently, a few local venues have popped up serving English language movies that are about 6 months behind their market release. Most of the tourist centers (Thamel & Freak Street) still offer pirated videos on TV's with volumes exceeding the comfort level. These places usually require you eat at the restaurant and the movie is included.