Compose simple shapes to form larger shapes. For example, "Can you join these two triangles with full sides touching to make a rectangle?"
Compose shapes to form others. For example, two right triangles can make a rectangle. Two right triangles can also make an isosceles triangle. Give kids some polyominoes, such as the T tetromino, and ask them to make a square or rectangle. Lots of puzzles can be tried at this point.
Notes: Khan Academy.
TODO: What manipulatives are available to buy? Maybe hand2mind has some.