In addition to the above...
You can listen to texts and dictate responses. ("Read texts", "Reply home in 30 minutes")
You can ask for movie listings ("What movies are playing nearby?") and reviews ("Is Catching fire any good?")
You can get sports scores and schedules. ("How did the Chicago Bears do last night?", "When does Manchester United play next?")
You can get restaurant and business locations and reviews ("Where can I find Ethiopian food near here?")
You can ask for calculations and conversions. ("What is 762 times 43?", "How many inches are there in 1.44 meters?")
You can ask Siri to define words or terms for you. ("What does eviscerate mean?")
You can ask for directions ("What's the best way to get to work from here?", "Take me home") and traffic conditions ("What's traffic like?")
You can ask for weather conditions. ("What's the hourly forecast?", "Will it be nice outside this weekend?")
And if all else fails, just engage Siri in conversation. The responses range from completely missing the point to hilarious.