There are many, many funny things to ask Siri.
Tell me a joke.
How much wood can a woodchuck chuck?
Am I cute?
However, there are a few things you should NEVER ask Siri.
My kids were playing with my wife's iPhone and asking Siri all sorts of questions. They were having a blast.
My daughter eventually asked Siri if she would refer to her as Petunia. Apparently, Siri doesn't recognize the difference between my daughter's voice and my wife's voice. Siri now calls my wife Petunia. Although amusing, there are times when that is not the most appropriate nickname.
You should also never ask Siri to call your boyfriend or girlfriend in front of significant other. Siri tends to make assumptions that may not necessarily be true or helpful to your relationship.
Here is a list of 7 things you should never ask Siri. Some are funny. Some, like the one about 911, are pretty serious.