Strangely it never occurred to me to use my harp as an opportunity for Intensive Interaction. Until this morning, when R. started to strum on it!
A major advantage of the harp over the piano is that if you kneel down on the opposite side of the harp you can get really good eye contact with the other person (through the strings) - overcoming the usual problem with the piano of sitting side-by-side. So I copied some of his rhythms, sequences of notes, etc. as well as experimenting with some rather unorthodox techniques including rubbing harp strings with our fingers and thumbs to make them 'squeak' as well as patting the strings with the palms of our hands. We both had lots of fun!