People don't necessarily come to Phonsavan for the town itself, but rather for the mysterious Plain of Jars! You should really put this on your list during your trip to Laos! The scenery that lines the route is magnificent, you can pass through several villages and I loved the motorbike trip to the plains.
We were advised not to visit all the sites; a jar is, after all, a jar. So we opted for Site 1, the biggest of them all. The ambience is mesmerising and you won't be in a rush to leave. It's a mystical place that I really liked.
In the Old Capital there's a superb Stuppa covered in vegetation and, if you want to see the three sites, they're not far away and well sign-posted. Turn right at the first sign you see - Site 1 is 2km away. The entry fee is 10,000 Kip. One piece of advice: take a fleece! The plains are on plateau at a high altitude and it can get cold!