The "Colonnes de Buren" ("Buren's Columns") at the Palais Royal seem to invite kids to transform themselves into human graffiti.
This is probably my favorite spot in Paris and I submitted this picture for the A Moment in Time collage in the New York Times Lens Blog a few months ago. An odd favorite place I know, but I think I love it because it's whimsical and unexpected--words that are not used to generally describe Paris.