Vang Vieng is a town alongside the Nam Song river. It is located north of Vientiane. Vang Vieng used to be famous for its river tubing experience while being high. All of that stopped after loads of tourists drowned into the river.
There are several viewpoints, caves and waterfalls to explore. Everything is easily accessible by bicycle and can be done in a day or two. If you are not in an adventurous mood, you can just enjoy a beer alongside the river!
Unless you want to get some rest, I do not see the point of staying more than 2 or 3 days. It is touristy as hell and there is not so much of local life going on!