Luang Prabang
This sleepy town is the perfect spot to stop and watch life pass you by. As I am sure this community has been doing for its entire existence. Temples from pre-communist regime remain completely intact (and perhaps exactly as they were left). The mists rising off the Mekong are a photographers dream - attached photo of the view from my guest house. Walking with the monks each morning and visiting literally 100's of wahts passes as many days as you care to spend.
Quick Tips:
Take the time to trek into a Hmong village in the hills while also walking past one of the many waterfalls in the area. You can also purchase amazing silk tapestries about a ten minute taxi outside of town. Money means basically nothing in this town as everything compared to the Pound or USD is almost free. When you get to the bank you'll understand what I mean when they pass you massive stacks of currency. Beer Laos is quite possibly the best Lager in all of Asia.
Best Way To Get Around:
Transport down the river can be made on either side (Laos or Thailand) but I made it in Laos. Took a fast boat down, which takes one day as opposed to three but its up to you how much time you want to spend (Pak Beng is halfway down and is pretty impressive for a small small town - great hospitality but stay away from the lao-lao.
In Luang Prabang you can walk almost anywhere and if not pick up a tuk-tuk they're really cheap.