Worries about a growing disconnect between people and the outdoors continue, but at least at the 394 US national parks, nearly 300 million people visited in 2009, continuing a rising trend in recent years.
In order to prod potential visitors into action in 2011, the National Park Service is offering 17 free admission days, covering five time periods. Other fees, such as for camping and tours, will remain in effect, but many parks are expected to discount some charges. A number of national parks always have free admission.
The Martin Luther King Jr. extended weekend was the first bargain, but you can give your audience a heads up on the others:
- April 16-24: National Park Week
- June 21: first day of summer
- September 24: National Public Lands Day
- November 11-13: Veterans Day weekend
- National Park Service fee-free program;  Jan. 5, 2011, press release;  park locator;  visitation history. 
For much more information on National Park visitation trends, and the broader topic of engagement with the outdoors, see the TipSheet of Aug. 5, 2009.