I occasionally get into the mood to just noodle on heads because I like trying to catch expressions. Animals are always a lot of fun to draw; they constantly surprise you with the strange differences in their anatomy from humans. And to be honest, they can be a relief because you don't have the pressure of catching the exact resemblance to a specific individual like you do when you draw friends and other people (unless you're drawing someone's beloved pet, then it's worse!).

Zoos are great for this sort of thing, if you have the time. Your own pets are great for drawing anytime, of course, but that's for another sketchbook page! Books with good color photos are very useful for doing studies to learn the anatomy, proportions and colorations. They have the convenience that you can draw from the images at home in a comfy chair and take as long as you like over your drawing. There are disadvantages, though; you don't learn the anatomy as well since you can't change your position and examine a weird part from another viewpoint, and frequently odd angles or cropping in the photo will cause you to misinterpret what's happening with various parts. But let's get real, photos are a great resource and give you access to animals you'd never get to draw otherwise. Not to mention that they don't move!

Use a medium you feel comfortable in, and just let go and enjoy yourself. You can learn a lot about the animal from each stage of your drawing; the initial rough layout will make you learn their unusual proportions and the relationships between various features, filling in more detail makes you consider hair patterns, light values and colorations of the animal. Feel free to exaggerate and embellish, too, because to do it well you really have to know the animal's unique characteristics!