Perspective is hard. I like to start on basic understanding of spatial planes as early as I can. This project is good for grades 3-5, and produces great results with basic shapes.
I start by showing students a picture of a landscape and having them pick out the foreground, middle ground, and background. I point out that things in the foreground are at the bottom of the page and look bigger, while things in the background are at the top of the page and look smaller. Middle ground items are in the middle, obviously, and are smaller than the things in the foreground, but bigger than things in the background. Some students have a difficult time grasping this, so I ask them to point out things in the foreground, middle ground, and background that they can see from where they are sitting. This usually helps.
Then, I demonstrate how to draw a pumpkin patch on the whiteboard and invite students to draw with me. We start with 4 irregular circles at the bottom of the page, going off the paper. Then we draw a few more, slightly smaller, going behind those four and ending just short of the right side of the page. And then another row of smaller pumpkins behind those, ending before the previous row. And so forth, and so on until we have a pumpkin patch that goes deep into the middle ground (or, up to just above the middle of the page.) I then demonstrate how to draw things that may be near a pumpkin patch, like a barn, scarecrow, trees, fence, etc… in the middle ground, just behind the patch. I encourage students to come up with their own fall scene.
Then, students will color their scene, making loopy vines and leaves between pumpkins, but leaving the ground white. They should also leave the sky white, but color everything in the picture. (I always demonstrate coloring the tops of pumpkins yellow like the moon and bottoms darker orange, so it will look like the moonlight is shining on the pumpkins.) We then paint the entire page with black watercolor paint to make the painting look like night time.
These pictures are always so impressive and students are so excited to see their picture turn from day into night with the watercolor wash.