Here are some ideas for activities to do with elementary aged children involving plastic bags.
Wrap sacks around toilet paper tubes or paint rollers. Roll over wet paint-covered paper to create textures.
Cut into strips and weave together or braid.
Use rock or sand to test the strength of the handles or the bag itself. Measure weight by hanging the bag from a fish scale.
Estimate the difference of mass of plastic bags and paper sacks when tossed in the landfill. Calculate for a household per year, and for an entire city per year.