Here’s an enjoyable way to spend ten minutes giving your brain some exercise: The Eyeballing Game. The game/exercise asks you to modify a polygon to create a parallelogram and right angle, find the midpoint of a line, bisect an angle, find the center or a triangle and circle, and identify a convergence point.
My average score, the degree on inaccuracy, (as you can see below) was 4.01 (low is better). By best showing was bisecting an angle.
Think you can do better? Give it a shot: http://woodgears.ca/eyeball/