The difference between Vientiane and Luang Prabang is what strikes you first. Luang Prabang have been so well preserved and the city is now a World Heritage site. It has a beautiful mix of mountains, rivers, forests and temples. It was also the city where we saw most of the tourists. With a population of around 50,000 the city feels small and family friendly and it has a calm and very relaxed atmosphere. The buildings have a colonial style look.
A very nice half-day trip is to go to the Kuang Si Waterfalls 20 miles outside of town. The waterfalls a multi-tiered and there is options to swim in the lower levels of the waterfall.