I haven't been for years, but used to live in a Brussels suburb, and regularly visited Antwerp RLD.
If hiring a car is an option I would go for that.
The airport is on the right side of the city for a quick visit (I once did it in 20 minutes to the Antwerp RLD, but I would allow 35 mins). There is plenty of street parking just next to it. If you've only got 4 or 5 hours it's probably not an option, but having a car means Den Haag is only another hour up the road.
Brussels RLD and the area is a shithole, however living there was fantastic - fairly peaceful relaxed people, and amazing food and drink, with some great punting only a stone's throw away.