Siri can returna random number, letter, or word.
After reading Lex’s hint about rolling the dice and flipping a coin, I decided to see whether Siri can generate random numbers. It can have Wolfram do it. You can speak “random number” (which it interprets as “random integer”), “random integer”, or “random real”. You can also specify ranges, such as “random number between ten and 100” or “random real between 20 and 30”.

“random word” and “random letter” also work.

Source: Mac OSX Hints