CRL is a separate airport south of Brussels and can suit a Wallonia route. Do not book a BRU branch for a CRL arrival.
The official airport lists rental companies on site. The desk and vehicle location depend on the supplier; use the voucher and airport car-rental signs rather than the Brussels city name.
A car is useful for an Ardennes itinerary, rural guesthouses and stops outside the main railway network. For Brussels, Bruges, Ghent and Antwerp city breaks, compare rail with a rental: parking and city access can outweigh the convenience of keeping a car throughout the stay.
Compare airport collection with a later collection if you start with a city stay. Include the transport to a different rental branch and any extra parking nights when comparing the total cost.