There isn't much to do at night in Lumut besides do what the locals do. Have pulled tea and a late supper at the local coffe shops. If you are looking for other entertainment like karaoke, you'd have to go to the neighbouring Sitiawan
There are some small western themed "bistros" where alcohol is sold but at a higher price. Go to the city center and there are some bistros facing the sea where you can have a drink and meet some foreigners
Not very much to do in Lumut at night. It is basically the parking lot for Pangkor island goers. There is for westerners one pub/rest. called Jooks, which is not bad, not very busy though. Also Rockafellas, which is not bad, has free pool table but quiet except friday night, Barbaque. Also is Capri restaurant, which has the best food of the 3.