Any real or homemade instruments you can bring. Percussion is the best choice—tambourines, shakers, tin cans of various sizes, 5 gallon plastic drums, glass jars, bongos, sticks, etc. Instruments that make actual notes run of risk of sounding bad when various players of different abilities (or none at all) play concurrently.
Invite the group members to make sounds using the vast assortment of different objects and/or instruments provided. Observe if people play independently or try to play with others. The resulting noise can be good, or bad but funny, but sure to break the ice.