Whether an activity to kick off a large event or just as a simple line to start a conversation, people struggle to find fun and sincere ice breakers.
Because I feel artificial and struggle, sometimes I forget that this is also how most folks feel. Sometimes, it isn't important how terrific your opening as much as someone just needs to create one because then, with relief, everyone will start to participate.
My favourite ice breaker is to introduce an open ended question that either must be debated, considered or requires more than a one word answer. "How are you?" does not work because "Fine" is the standard response :-) "What do you do for a living?" does not work because either someone doesn't wish to be defined by their job or is now worried you're looking for one.
I have used - with success - "What brought you here today?" "Where did you get those shoes?" "This is a big crowd - do you know anyone?"
Of course we stay within our comfort zones when choosing a question - mine might not work for you!
For large events, I try to encourage folks to mingle and talk by giving them an excuse to do so. Like a glass of wine, an ice breaker at an event is a social lubricant. Some events, it's a group quiz; others I will pose a challenge.
Folks want to mingle; they just don't want to feel like it is forced when they approach someone. So make it easy for them by swallowing your own discomfort and finding a simple way to create an opening. Chances are, everyone will follow your lead.