I am a Welsh harp
This exercise, taught to us by Lou Spaventa, was around among Pilgrims trainers back in the late 70's of the past century.
Lou would use the activity as a warm-up at the start of a group:
Ask each student to decide which musical instrument they associate with, which instrument they want to "become" for 10 minutes.
Put on some orchestral music and ask the students to walk around the room.
When the music stops each person tells some one near them what they "are" and the why they chose that way.
Repeat this procedure 6 to 8 times, sufficient for each person to develop their feeling of empathy with the instrument they have chosen.
To turn Lou's activity into a writing exercise do the following:
Ask each student to choose an instrument they can " become".
Randomly pair the students. Ask them to
a) tell each other which instrument they "are"
b) go and sit down away from their partner.
Ask each student , writing as their real self, to produce a one page letter addressed to their partner as the instrument chosen. The letter will look like this:
Dear Double Bass,
The content of each letter is up to each student- it is not every day you have the chance to write a letter to a piccolo!
When the writing phase is drawing to an end ask the partners to rejoin each other and exchange letters.
Each student goes back to their place and replies to the letter received, in role as the instrument.
The partners come together again, exchange letters and discuss the experience of this kind of writing.