What kind of cookie breaks easily into multiple pieces when dropped?
See if you can find a cookie that is both thin and crisp -- the thinner the better. Minor damage will be more destructive on a thin cookie, while on a thicker one it would just cause some crumbing.


A tuile came immediately to mind as one that gets relatively fragile once it cools. They come in wafers or rolled into cigars. (Pepperidge Farm sells a version, Pirouette, that are filled with chocolate -- but filling might buffer the cookie too much.)
A pizzele can also be pretty breakable, but when I've seen them in commercial packaging they are a softer version.
A fortune cookie might be too robust to break on a six foot drop, depending on the angle of impact.


If you decide to make your own, do the opposite of advice to prevent crumbly cookies: use a "softer" flour (cake flour rather than all-purpose, which has less protein), use too much flour, use not enough fat, overbake, and use a dark cookie sheet. These cookies aren't likely to taste quite as good, though, so if your secondary goal is a tasty treat it might not work out!