Set your child up with a sheet of blank paper (or you could make up a simple document like the sheet below), and a mirror. I think a felt tip pen works best for this project as the lines are sharp and good for capturing finer detail.
Ask them to look closely at themselves and draw what they see. It is a good lesson in observational drawing, and makes a lovely keepsake.
Once they have finished, ask them to describe the drawing and a talk a bit about themselves for you to write down.
I worked on this project last year with a group of 4 year olds last year, and this was one of my favourites.