Spa is a must do for anyone serious about their driving. You guys will have a great time with Jonny et al. from Bookatrack, they are a nice friendly bunch and you'll have some good banter, good food and of course being Belgium, some excellent beer!
Spa is an easy four hour drive from Calais too (we did it in three hours once), so it's not that far.
I will be going back there again, it's hard to believe the last time was 2006!