A list of locations that rent bikes:

https://www.cyclinginflanders.cc/plan-your-stay/bike-rental

Bikes for rental in several locations in Flanders and Brussels:

https://www.cctbikerental.com/location/bike-rentals-flanders-ardennes/

Road bikes for rental from Livelo:

https://www.livelo.cc/collections/brussels-road-bike-rental

https://www.livelo.cc/collections/road-bike-rental-ghent

https://www.livelo.cc/collections/leuven-road-bike-rental

https://www.livelo.cc/collections/antwerp-road-bike-rental