A nice trick I like to use is the following, but mathematically not exactly the same as the above options. It's very fast though, and versatile.
Activate pencil tool Hold mouse over rectangle (or any other desired face) until it says 'on face' from then on, hold shift. draw any desired shape, aligning with points or lines already in the drawing (NB: aligning with a line is not the same thing as aligning with a point!) after finishing the shape, release shift.