One of the most useful things you can learn how to do is to take a quick screenshot, edit it, and use it in documents, blog posts, websites, presentations, etc. There are lots of ways to do this; here's one that we use on Chromebooks all the time.
Awesome Screenshot is in the Chrome Web Store and works on any computer running Google Chrome, not just on Chromebooks. Try it out!
The above video was created on a Chromebook, using Google Hangouts On The Air and WeVideo.