With the 3rd and 4th graders, I did a lesson involving perspective and vanishing points. A few weeks ago I did a project with a simple still-life and using one color, and I noticed that most of the kids got frustrated with drawing a 3-d box. This lesson hopefully will give them a bit more insight into perspective.
The first step was to have them draw a flat horizon line on their paper and a dot vanishing point in the middle of that horizon line. Then they drew some simple 2-d shapes in the "foreground" like squares, circles, stars etc. With a ruler they then connected all the edges and corners with the vanishing point they drew earlier.
This resulted in a lot of "railroad track" or "laser beam" looking objects that go forever, so I showed them the trick of adding a back to their shape and erasing their vanishing lines.Many of the kids ran with this and made pictures that looked like train tracks or piers jutting into the ocean. I also showed them how to make their names in 3-d as well.
I was amazed with how much effort they put into these drawings and how quiet and concentrated they were.