It is a nice place to enjoy the sun when the weather is beautiful in Brussels! During summer you get to see a lot of festivals going on there and the park then gets really crowded and you can lay in the grass whilst enjoying music or just looking around the statues that are in the park.
It is one of the rare parks in the centre of Brussels, so don't miss it (unless it's raining obviously), because then the magic would just go away, like that.
Take a towel or something to sit on, because belgian grass is easily wet (maybe because it rains a lot?)