Prés de la table is a technique which gives quite a distinctive, metallic, guitar-like tone. It is created by playing near the soundboard, as Miriam demonstrates.
Miriam also demonstrates some more unorthodox techniques for playing the harp.
You could try hitting the harp with the palm of your hand, which sounds a little like a gong, or you could run your fingers up and down strings.
Playing with the finger nails is also effective, and gives an oriental sound - a little like a Japanese Koto.