Take anything with a hard, uneven surface (in this case, coins). Stick them under a sheet of aluminum foil. Press around the outside first (so the object will likely stay in place) and press all over the surface to create the imprint.
Alternatively, you could make this a larger project by first gluing yarn to a piece of paper or cardboard, either in a pattern or randomly, then pressing the foil over that. (Use thick yarn, or even twine for this).