Each year as an introduction to portraiture using a grid method, I have my Art students enlarge and draw what appears to be a totally non-objective image from a 1/2" square onto a 2" square. They all use the same type of mechanical pencil to help keep the shading consistent.
The kids don't know what they're drawing until all of the squares are put together--like a big puzzle--to form a picture!
Intro to this exercise involves doing value scales and also doing some preliminary drawings that simply involve copying various shapes.
The squares are coded on the back, and when students complete all of the 2" squares, they glue them onto a gridded sheet of paper to form a large, mosaic-like portrait.