If you are wandering along the canals, Gerald the Devil's castle is an interesting picture. I would have enjoyed seeing inside, but it was closed to the public (June 2017). Overall, it is still an interesting bit of architecture.
This is worth a visit but Gravensteen is much better. This castle can only be viewed from the outside as it is not open to the public, but it's grand size and white stone walls make for some good photos.