For about $45 you can purchase an iPad stand that will allow you to use your iPad as a document camera. You can use the Camera app to show work to students just as you would any document camera. Or you can use an app like Stage to record and annotate over live images captures with the camera on the iPad.
When you pair this stand with AirPlay or AppleTV, you can project these images to your students in class.
This iPad stand can be moved around your room to a lecture stand or a student desk. That way you can teach from any location in your classroom and share your own images, student work, or have students share ideas with the whole class.