You could have them work on a quilt for "Quilts 4 Kids". They could learn to sew a simple 4 patch quilt from a kit that you send for and they in turn could make it a service project for the troop. www.quiltsforkids.org
Pillowcases are great too, and could also be used as a service project for a local women's or homeless shelter. They have a really easy pattern from www.allpeoplequilt.com, search for "tube pillowcase".