One of my favorite drawing exercises is to pick random objects and draw them in pencil, erasing as little as possible. The objective is to practice my observation skills by trying to get perspective and proportions right since the first pencil mark. The amount of corrections varies from drawing to drawing but I feel I always erase much less than I would a few months ago, and with much better results.
Croation sailboat made of rock and colored glass:
Here's how it looks like in real life: