Have a beer Lao on the Mekong. It is available about every 200 feet on the island, and big bottles for 10,000 Kip. Head toward the west part of the island for a great view of the sunset.
There are a few bars along the main street that is popular with Westerners, and there is even 1 that shows English movies and comfortable seatings (pretty similar to Vang Vieng). If you prefer a quieter drink, go to one of the bars with a pool table where you can match your skills with a local.