Play-dough doesn't get simpler than this!
-- You will need --
-- STEP 1 --
Dump two (2) 16-oz boxes of corn starch into a plastic container.
-- STEP 2 --
Add an entire container of white foam shaving cream. I got the one shown at the dollar store.
-- STEP 3 --
Let the kids mix the corn starch as you add the shaving cream and be prepared with molds if desired.
Image credit: www.instructables.com