Drawing exercise made fun

Drawing ellipses is a very good drawing exercise, but can also become very boring. What I try to do to lighten up the exercise is making a subject out of the ellipses. For instance: donuts. They come in a very basic form, are therefore easy subjects, they can be bought mostly everywhere and, last but not least, after finishing your exercise you can eat your drawing model! Is there any other hobby with such an advantage?

Sketch

As I was inking the drawing of the Citroën HY an idea popped up in my mind about how that vehicle could be used. It can basically be used for almost any kind of work. For instance delivery of goods or transporting people from one place to another. Or, as I sketched above, a tool for two brothers working in the publicity industry. It was great fun making this. For me, not for the two brothers, who aren't having a good time at the looks of it.
Anyway, I can see some elements that aren't quite right yet. Some things are out of proportion or missing. There's a lot of work to be done. I'll post the drawing in a couple of days/weeks when its ready.