http://www.jeugdstadion.be/index_english.htm
A cheap municipal site in the town of Ypres.. just a few hours from Zeebrugge. hot showers at the push of a button. 10mins walk from the town centre where at 8pm each night they hold a memorial ceremony for the braves of World War One. We use it every year for a week or so at a time and find it more than adequate for our needs. Overnight, I would imagine it would be ideal. In Ypres, you can sample their local Ypra beer on tap, or Leffe lager... hmm.
The site, you will almost certainly find as cheap and can book online in advance. There is a 25euro depsit for a tag to allow access.. refundable on depart. Electrical hookup at the lower rate, but ample for running fridge, lights, tv and dvd/sat and 500w heating or electric hot water. 10 spots in a bullring shape. The site is next to a sporting complex that has a pool, gym, running track, crazy golf (not sure when the golf is open), small skateboard park. English spoken by staff.