One of the climbing schools I used to go to frequently was Freyr in Belgium. Although from the outside Belgium seems like a flat country without much interest for vertically-minded adventurers, it hides well some jewels for the initiated ones. Freyr is one of these spots, rightly named for the Scandinavian god of productivity, sun, and rain. It is one of the biggest climbing areas serving the North of France, all of Netherlands, and Belgium. Even people from Germany and Luxemburg show up for some fun. There are several separate rock formations, and over 300 mostly bolted routes.
However, one has to be warned that climbing here has started in 1930ies, and that means that routes are (extremely) polished, especially in the easier grade. Moreover, Belgians are proud in having a pretty spicy grading system, so don't get disappointed if you don't send the same grades here as in the more Southern places. Freyr is in a way similar to St Llorenç near Barcelona, it needs some getting used to to start appreciating climbing there, but there are loads of very good climbs when you get started. Below myself on one of those jewels, Heroine, 7a+: