Drive off the Eurotunnel and take the A16 east (last stop Vladivostok). Be aware of the variable speed limits along the A16, which also change in the rain. The limit is 130klm which is 80mph. French drivers have a tendency to (hopefully) indicate left then immediately pull out into your braking distance, so just be alert to this. It will take about 1.5 hours from Calais to Ghent mostly just on this motorway - mostly 2 lanes but less traffic than the UK. Park in Ghent as per the photo - it’s outside the emissions zone in an underground car park with music and green lights to indicate empty spaces. The centre of Ghent is about a 20 minute walk - turn left as you come out of the car park stairs. Not many cars in Ghent, a few electric ones but mostly bikes so be aware crossing the road and trams. Easily do-able for a day trip. The castle is well worth a visit and the 40 min river cruise.

Looks like a great trip.