I've stayed a good few times at the camping municipal in Massat. The above poster mentions pools and kids, and that's fine, but this site has nothing, so no kids, just peace and quiet. It's not on the main Ax to Andorra road, so it's not full all the time, the village is really nice, especially if you're an old hippy!
There you go - lovely site next to the lake and river, on-site canoe hire, and right underneath the Sinsat crags. John and Anne Arran of the UKC parish live in the village if you need any advice about the climbing, and Ax les Thermes and Tarascon are nice towns for sightseeing and mooching.
ps - if you use Google maps on the satellite setting, you can get a good view of the campsite.