IMHO, it is hard to find a domain that is challenging, useful, and small. But a couple of examples listed above are pretty good: 1) Write your own web server, blog software, etc: By leveraging existing libraries (or not!) you can get a lot of effect for very few lines 2) Script your daily chores: It does not matter what language you write the “scripts” in. Choose your current favorite and get a feel for how you can interact with the OS