Day spa??

If you're willing to travel, there is one in Grimbergen.
http://www.thermae-grimbergen.be/NL/Home
Otherwise, Castle Club in Wezembeek has pool and sauna in the basic fee http://www.sports-valley.be/

THere is a pool here: http://www.sportcity-woluwe.be/francais/index.html

I think it's also possible to access the wellness center of Dolce La Hulpe on a day-basis, it has both sauna, hammam and pool - but you'd have to confirm it with them
http://www.dolcelahulpe.com/activities/fitness