You can do anything you want. They're basically blank webpages you can add HTML to. For an overview of how to add and edit custom pages, see this article.
If you aren't an HTML expert, don't worry, we've put together some code snippets you can copy and paste to add the most commonly requested pages.
There are also several sites which will allow you to generate a widget from an RSS feed such as a blog. Once you work through the creation process, these sites will provide you with a snippet of code you copy and paste into your custom page (make sure you are in code mode, not visual mode, by clicking the </> button).