Polyfoam ("styrofoam") and balsa wood are often used in breakaway or stunt props. Chairs and table can have their legs removed and attached with a touch of glue, so that they are barely held in place and the lightest blow will dislodge them.
Make sure to use a saw and properly score the prop.
Remember that even with breakaway props, actors are still striking each other with objects, so use caution.
