To celebrate the approaching summer, incorporate a rainstorm activity in your music therapy session.
I've seen this used a lot with kids as a socialization activity, but that is only the tip of an iceberg of potential. Use this in large counseling groups to represent a "passing storm" of loss, depression, and anxiety. Use it to facilitate a healthy release of guilt, regret, and sorrow. Understand that in a counseling setting, this activity can become something very emotional and significant in moving past an unpleasant time.
However, on that note I must give some words of advice... (1) Take your time in building the rainstorm. When we stand in front of a group, it is easy to rush. Remind yourself to go slow. Give your group time to sit in the moment as they listen to their storm. The moment can be a very special one, but if you don't give your clients a chance to reflect on this, then you lose the therapeutic element.
(2) Because this activity is so adaptable, you must be mindful of age appropriateness. Don't assume that your adults will enjoy playing the thunder tube. Kids may make a rainstorm with small percussion instruments (ocean drums, thunder tubes, etc.), teens may make a storm with their bodies ("shhhh," rubbing hands together, patting thighs, etc.), and adults may use larger drums and various percussion instruments (bass drum as thunder, etc.). Consider your own group and decide what you can do to get the most positive response.
The video below can be an inspiration for your own session. In the first two minutes, the choir builds a rainstorm without any instruments at all. Since I came across this link, it has been bookmarked in Firefox. It is a performance that should not be forgotten because of its potential for use therapy.