The documentary film has moved from being solely a denizen of the history and science classrooms to becoming a full-fledged art form. Documentary filmmakers like Michael Moore and Morgan Spurlock have pushed the boundaries of the form to make it appealing to a broader audience, and now documentaries cover topics ranging from air guitar contests, to steriod use in sports, to the US culture surrounding food.
Consider current trends in culture and society. A documentary is a great way to explore the implications of a new trend or to examine behind the scenes of a current development. Examples of documentaries that have done this with varying degrees of success are "Bowling For Columbine," which took an in-depth look at the causes and implications of school violence and US gun laws, or "An Inconvenient Truth," which looked at the possible end results of global climate change. Both documentaries did well at the box office, making many times more than their cost. While both of these documentaries tend to be serious, they engage the audience in a fun and entertaining way. Find a topic that will capture the interest of the audience and make them look at something familiar in a different light.
Choose something that isn't widely covered in other outlets. Chris Rock, comedian and actor, made a documentary called "Hair" that went behind the scenes of the trend among African American women of modifying their hair with extensions and chemicals. It was an obscure topic that shed light on a subject unexplored by the mainstream media. Alexandra Lipsitz's "Air Guitar Nation" is another, similarly obscure topic that was made into a successful documentary. Find subjects that haven't been previously explored, and you'll be well on your way to making a fun documentary that people will want to see.
When you're devising a topic for your documentary, the narrower you can get with your topic, the more fun you and your audience will have. If you want to make a documentary about the modern fur trade, for instance, don't try to cover everything about it. Choose a few key individuals to focus on, and let the audience get to know them through the way you portray them on film. Narrowing your focus will also allow you to share details your audience wouldn't get in a more cursory look at your subject, and it allows you more freedom to personalize the film and connect with the audience.
Keep it Light
Above all else, the key to making a high-quality, fun documentary is to keep the tone of the film light. While you may be tackling a serious subject, that doesn't mean the tone of your documentary must be serious. Morgan Spurlock's expose, "Supersize Me," took on a rather serious topic when he covered the health effects of eating fast food, yet he did so in a way that engaged the audience and kept it fun. Achieving that tone is the key to making a fun documentary.