We're just back from a trip to mainly Holland, which I'd highly recommend. The Dutch are really into camping and the sites we stayed at were laid back, very well run and among the best campsites we've been to anywhere.
Camping de Kooi
http://www.campingdekooi.nl/ is on the island of Terschelling, one of the Wadden Islands and a 2 hour ferry north. Terschelling is an absolute gem of an island - largely car-free, white sand beaches, laid back and great towns, restaurants etc. The Dutch know about this place (we were recommended to come here by a Dutch friend) but apart from one German car we saw we were the only other foreigners on the island.
Camping de Roos
http://www.camping-de-roos.nl is further south near Ommen and a 'natural' site spread over a large area beside a river. It deserves its place in the Cool Camping Europe book.
The interesting thing is that both of these sites are car-free. Obviously we took our van onto the site but we weren't then allowed to move it. This means that the pace of life slows down and you do as the Dutch do and cycle everywhere which is great.