So what do you include in your sketchbook if you aren't drawing from the infinite lists found online? Everything is the short answer. As an illustrator your interests need to expand past visual art, artists and designers. You need to be a collector of information and things. This means you must be interested in everything from politics to the environment, to pop culture, and even historical facts. Becoming a curator of information, will help you in creating those sketches. Be sure to be observing the world around you. Go to your local coffee shop, train station, or mall and draw. Draw the elderly man drinking a coffee, the tourists waiting for their 3 o'clock train, or the children running through the mall food court. All of these things show that you are observing and taking in the information surrounding you. It is this information which will allow you to convey the solutions to the problems proposed to you from potential clients.
Don’t forget, as an illustrator you are essentially a problem solver, an image maker and a designer. Your goal is to help your prospective client navigate their current problem, while producing a clear solution, using the imagery you create as the key communication tool. Whether this is through traditional mediums, such as oil paintings, or digital mediums using Photoshop or Illustrator your imagery must start with a rough piece. This is one of the reasons a sketchbook is so important. You're work will evolve and change as you weed out the less relevant concepts for your final illustrations. This is where your sketchbook will be used as a form of process or idea book. Whether it be rough sketches, composition drawings, or colour roughs, your sketchbook is where all of these ideas should start to take form. Showing this process in your sketchbook, gives those viewing it a look into your creative process, which is what universities such as OCAD U are looking for.
Finally, remember to use your sketchbook to explore and have fun. Although your expected to be using your sketchbook for rough work and concepts, you should also be using your sketchbook as an outlet to explore new mediums, express silly ideas, and draw what you are interested in. A sketchbook can be an expression of yourself, where all ideas, good or bad begin. Don’t be afraid to use a strange new color palette on a silly doodle of a unicorn riding a dragon. Or to make a tape drawing out of the new colored tape you found in the crafts isle of the art store. Make sure you enjoy the time you spend in your sketchbook. Schools, like OCAD University are looking to the sketchbook to see your thought process, and your experimentation. They want to see someone who is constantly observing and consistently creating.
Use your sketchbook as a tool to create, to observe, to retain information, and to experiment with new techniques, tools, and to express yourself and your works. Using some of these will create an excellent example of your work and yourself as a creator.