When you're bored, drawing is the best time for your creativity to come out of hiding, because being bored means you have the time or else you wouldn't have got bored in the first place, so if you are at work, then ditch it for an hour and start to draw, lets face it, if your work or real life job bores you, then drawing seems like the better option.
So what do you want to draw? you could draw anything that pops into your head, a fantasy creature,a fairy or a knight in shining armour, it's up to you to entertain yourself whilst sketching away at your desk, so start with what you really like to draw first and work backwards into what you have trouble drawing, trust me this is a great idea as you start off drawing what you are familiar with, you can then start to draw with your beginners confidence on your difficult subjects.
I tend to have trouble drawing imaginary details on swords and shields and that's why I heavily plan intricate designs out well in advance, so that when it comes to drawing the permanent pencil lines in, it looks right, that's the way of the ninja artist!
Also instead of using your imagination, why not use your surroundings as inspiration and start to sketch what you can see, this is a good option as you can draw anything in your room or house that seems like a good idea, I like to draw wooden things like chairs and unites, because often there is the wood grain which I love to draw and have fun with that,similar to drawing trees with all the bark that is like sketching random lines of flaky bark.
The people around you or on a walk about is another great idea, quick sketching like this can really be a boost to your artistic skills, as you are drawing from life you can pick up great body poses and stuff from life that is happening and in a daily motion captured forever in your sketchbook.
Drawing when you are bored can happen any day, not just at work, at home is an ideal option, because you are relaxed and in a comfortable surrounding (usually) I tend to chill out and watch a movie with a sketchpad on my knee to capture and ideas or even listen to some music that will help give my sketches energy and jazz up some new ideas on the paper.
Try drawing when you are bored and see what results you can generate, you'll be surprised at what drawings you can achieve when you put your mind to it.