Generally low key. Eat, drink, meet people at the local bars, sing at the Karaoke, see a local Thai boxing match. Koh Lanta isn't about a rollicking nightlife but you can have fun. After 9 - 10 pm, most shops will close in Saladan Village, but restaurants often open till late.
A lovely place to end the day is on the nice Klong Nin Beach. Try "Richey's" tree house", cool atmosphere, good music and a warm welcome. Beach bars are both plentiful and popular, walk along the beach to find them, usually they'll stay open as long as they've got customers.