Make instruments out of junk.
Various junk e.g. bottles, tissue boxes, youghert pots etc. Elastic bands, rice, clingfilm etc. Stong glue, sticky tape, papper clips etc.
Beavers to use junk to make instruments e.g. put some rice into a jam-jar do up the lid and you have a shaker. Put some elastic bands of differeing strength AROUND A SHOE BOX FOR A "GUITAR". Let them do what ever they want. Ask Beavers to demonstrate their instruments to everyone.