Don't worry, Dave- they have to "invent" an instrument- they can be inspired by existing instruments but must use no part of any. So, wire is ok, guitar strings are not. I get lots of "boxophones", long-neckd strings, pvc pan pipes, glass bottle pipes and many, many plastic drums. They compose a "recycled percussion" piece in small groups and may or may not notate, depending on how much time we have (this year we didn't). They then present their compositions at an assembly or in front of a kindergarten class. This year the time was so constrained we created whole-class compositions- each child self-selected the category he/she fit into and each group created a rhythm or melody pattern. Then the whole group came together and had to figure out how each part fit together, how to begin the piece, and how to get it.
Tomorrow we present the composition at their last school assembly! Next week, we'll sing our favorite songs- Hi ho the rattlin bog, Anne Boleyn, All God's Critters, Drunken Sailor, John Kanaka- stuff like that!