Some of my favorites are:
-the rectangular end of a chopstick, round end too.
-a ribbed marker cap, for making a lined pattern around the edge of anything, it makes a great edge.
-a large bolt, or nail head
-a sharp pencil, both ends
-a clay tool that has a gently rounded end that is narrow (not flat like a popsicle stick more like a field hockey stick)
I am a gardener and have made up all sorts of simple and complex flower shapes with a pencil, both ends, and the last item mentioned, the clay tool. Also, old costume jewelry, which kids delight over.