Question & answer
Audience submits and up/down votes each other's responses.
People in the audience submit responses and up/downvote each others responses. The more important issues float to the top allowing the presenter to tackle the most important issues.
More types of activities
Competition
Ask a series of questions and declare a winner at the end for the person who scores the most points.
Multiple choice
Ask a question and people choose a response from a set list of answers.