Turn a paper plate upside down and draw a dot in the center. Help the children draw lines from this dot to the edge of the inner part of the plate, then one more line along that edge to create a bunch of triangles. Have the children color each triangle and count how many there are. Then cut out the triangles along two edges and bend them up so it looks like a crown. The children can then add jewels or glitter to their crowns and color the outer edge of the plate.
Let the kids create a sailboat using two paper triangles for the sails -- one large and one small; a half circle for the boat; and a long, thin rectangle for the mast. They can glue each piece onto another sheet of paper, with the half circle at the bottom, the mast sticking up in the center, and the sails on the mast. Then they can color the sailboat, or even add a water scene, using paints or crayons.
Give the students a variety of triangles -- large and small -- cut out of sponges. Set out paints and a piece of paper for each child and let them create a picture using the different triangle shapes and different colors of paint. Or let them create a collage with triangle-shaped cutouts from paper, pictures and fabric. They can glue the triangles onto another piece of paper to create a collage.
Introduce a three-dimensional shape by having the kids create cardboard pyramids out of triangles. For each child, cut four identical equilateral triangles out of thin cardboard, adding small tabs on one side of each triangle so they can be glued together. Have the children glue each tab to the tab-free side of another triangle to create a pyramid. Let them dry before allowing the children to cover the pyramids with glue and sprinkle them lightly with sand.