Since Lankayan is a very small island, there's not much do to except diving. You can also walk around (in about 10 or 15 min. ;-)), go snorkelling, having a sunbath or - if you are lucky, watch young turtles getting out of their eggs or turtles laying eggs on the beach.
There's one diving school on the island which organizes 3 dives a day on different dive spots.
When you are going to Lankayan island you'll make a stop in the Sepilok Rehabilitation Center which is a wildlife station mainly for sick or orphaned Orang Utans. But there are also a lot of other animals, f. ex. macaques, deer, snakes, bats, rhinos, tigers, flying squirrels, huge insects
There are several resorts close to the Rehabilitation Center.
There's no night life on Lankayan. You get your dinner and afterwards you can have a beer or two, watch the stars or talk to your diving collegues.
Or you can go night diving instead which is really exciting. There's a lot to see since during the night other creatures come out to feed.
If you are in Sepilok, you can do a nightwalk with a national park ranger - quite a weird but very impressive experience...
We saw a hornbill bird, flying squirrels, a mouse deer, a big green pit viper. some big insects (mantis, giant ants...), giant geckos, bats and a lot more.
The ranger told us to follow his footsteps because the branches hang very low and on the branches there could be snakes or other dangerous animals... ;-)