There's already people here thinking about stuff like this. So, I'm cross-posting for folks here to read their ideas and post their own here.
On my end, I don't have an answer off top of head but do have ingredients. As in, it should be one or more projects that touch on various parts of the language: use of scalar and compound datatypes; different structuring a la programming in the large; different types of control flow; error handling; I/O (esp files and networking); concurrency and parallelism; wrapping existing code; anything that highlights language-specific advantages (eg macros for building DSL's). Probably best to have a few, small apps plus maybe one, mid-sized one.