"tell me a joke" (e.g. "The past, the present and the future walked into a bar. It was tense.") and the others work for me. What happens when you say "what can you do?"? Should bring up a list of various options.
One thing (this happened to me initially) was that I didn't have the language settings exactly matching (I had the voice as English (United Kingdom)). In that case I didn't get Cortana at all, though the screen design was similar. All settings are matching now and it's working great. Mostly...