Opposite hand drawing.
This is an old staple of drawing teachers the world over and requires only that you have two hands. If you only have one hand, I guess you get to skip this exercise, or maybe you'll be made to draw with your feet instead. However, if you happen to have two fully functioning hands, then it's a simple formula:
If you are right-handed, put the drawing utensil of your choice in your left hand and commence drawing. If you are left-handed, put that drawing utensil in your right.
As I mentioned, opposite hand drawing is a pretty common exercise and is definitely a classic. While it might be a mystery to some as to why anyone might bother, there are several good reasons to take on the task every now and again.
First off, let's suppose that you, as the owner of two fully-functional limbs, one day find yourself to be the owner of only one. This could be for any variety of reasons including amputation (intentional or otherwise), mangling due to attempted garbage disposal maintenance, or good old fashioned stroke. And let's also suppose that the limb you've managed to lose proper use of is the one you also happen to use to draw and paint. Given that drawing and painting is your livelihood (if it isn't you likely wouldn't have ever done this exercise, anyway), it looks like you'll have no choice but to employ the use of your other hand. If you've done some opposite hand drawing, then you've already laid the groundwork for overcoming this little obstacle.
Don't think it could happen? Well, I've got two words for you: Frank Frazetta. In the early 2000's, Frank Frazetta suffered a series of strokes which left his right hand useless. Not being the type to give up, he simply started drawing with his left hand instead. I doubt, somehow, that it was the first time he'd ever done so (though I could be horribly wrong and if you know different, please refrain from blabbing about it out as I'm trying to make a point here). If it could happen to Mr. Frazetta, if could happen to you. It's worth being prepared.
Doing any activity repeatedly builds pathways in the brain that, over time, improve one's ability to do said activity. Drawing with your opposite hand is no different. While I'm not saying that it's important that you become equally proficient with both hands, it doesn't hurt to have laid down a bit of a foundation to allow for a little insurance should something evil befall you.
Secondly, as with any other drawing exercise, opposite hand drawing can improve how you interpret what you see. It can help you more effectively translate the things before your eyes and push them out your pencil. Drawing exercises are practice and practice not only sharpens skills but helps keep them sharp. Last time I checked, that was a good idea. I know it's trite and obvious but it still bears repeating, because drawing, in my experience, isn't exactly like riding a bicycle. Such a skill can deteriorate. While I can't say as to whether the ability to draw can ever completely go away, I'm sure like me, you don't want to find out.
A third reason for such an exercise is that it's a change-up to your routine. As I mentioned before, change-ups are an important part of a complete breakfast. It keeps your practice from getting boring, and adds a new element to the mix. As tempting as eating cornflakes every day might be, it can be helpful to one's spirit to try something new. Eggs and sausage perhaps? Oatmeal? Leftover pizza? If nothing else, it'll make you realize just how easy you have it with your dominant hand, and the relief at getting back to your normal m.o. will be like a mental vacation.
Whatever you do, should you be tasked to draw with your opposite hand, don't avoid doing the exercise. I could have gotten away with it, personally, as I'm left-handed. When my professors in school told us to switch, I could have kept on going as I was only one of two lefties in my class and my professors didn't exactly keep track of who was a southpaw and who wasn't. While every once in a while, I would test their memory, I also found myself switching even when there was no mandate. The thing that was so appealing is that there was no pressure to do great drawings. It was pretty liberating to just sit back and make marks with but a meager hope that they land where you intend them to. That kind of liberation is pretty fun for me, and I can't say that I always have as much freedom in my day to day work. So, I enjoy it when I can.