Note: The site used for this activity plays music when you open it, so just be aware of that before clicking.
Music is an awesome therapeutic tool. In my in-person office, I have had to limit instruments used in the past because of shared walls with other therapists. (This is a hard balance in group practice – sometimes therapy is loud! Especially with kids. But other times therapy is quiet, and noise from other offices can be disruptive. If you are building an office space that may be used by therapists, please, please, soundproof.)

I recently found an online piano that you can play simultaneously with other people. There is an open platform, which has an ongoing chat feature that is not always appropriate. However, you can select “new room” at the bottom of the page, name your room, and get a unique link. De-select the option for the room to be public so that no one else joins your room, send the link to your client, and they will enter your private room without seeing the public space.
You can invite as many people as you want, so this activity works for groups as well as individuals. You can play the piano with a keyboard or by clicking on individual keys.
This activity is particularly great for music therapists.