As a project, develop a non-violent game using the Source SDK. For example, you may develop an application that gives a community of users access their personal collections of photographs.

One interesting feature to explore is the use of narratives, that is a kind of guided tour that gives a user an overview of the collection of photographs by means of a story, taking (in other words) the user by the hand in navigating the gane space.

For further reading I suggest, apart from the manuals and learning materials that come with the Source SDK, books on game development such as  [Luna (2003)],  [Gee (2003)] and  [Klabbers (2006)].