I've tried the chocolate bag idea before and find that it works better with wax paper. The brown bag didn't tear off very well. How about making a lantern with the brown paper bag? You can cut different shaped holes or pierce the paper with different patterns.
My boyfriend's sister is a teacher and brings home tons of schoolwork, so maybe a nice canvas bag with her name or initials on it? Last year a student gave her a handpainted container which she now keeps on her desk for treats on special days. I agree with SNK725 that any gift from a student would be appreciated.