Go to the casino if you must but in any case don't miss the geological museum where the owner gives a most informative tour in several languages.
Abbey of St Martin
Visit the Abbey of St Martin du Canigou near Casteil but don't take an expensive land rover trip until you've weighed up the walk from Casteil. If it's too much for you, on no account take the trip from further away. It will cost you far more and you can easily reach Casteil by bus.
zoo
There is a small zoo in Casteil that appeals to children.