There have been numerous reports of the demise of outdoor activities in recent years. These are sometimes interpreted to mean that there is also an ongoing decline in interest in all kinds of environmental issues, since people are losing touch with their outdoor roots.
At best, these reports appear to be half-baked. There have indeed been declines in recent years in visitations to some parks, and in participation in some activities. But many parks and activities have had increases in use — sometimes large ones. There are numerous starting points for looking into this issue in more depth.
Outdoor Recreation Participation
In the past three years, from 2006-2008, participation in dozens of outdoor sports has increased, according to the Outdoor Industry Association. Among those with substantial increases in that time period, and with a double-digit increase from either 2007-2008 or 2006-2008, are adventure racing, overnight backpacking, mountain biking, windsurfing/boardsailing, mountain climbing, sailing, snorkeling, snowshoeing, surfing, telemark skiing, trail running, triathlons, and wildlife viewing. Within the overall activity trend, there can be large differences by age group (with some age groups sharply negative even though the overall total is positive), so it could also be useful to parse the data that way.
Among the activities with negative overall trends in those time periods are several types of hunting and fishing, recreational vehicle and car camping, downhill skiing, water skiing, a number of team ball sports, snowmobiling, jet skiing, horseback riding, and bicycling. These happen to have prominent lobbies supporting them, or to be highly visible to a large percentage of the population, or to be expensive, possibly leading to declines during the recent economic bust. All those factors may be leading to more media coverage of those particular activities.
If you cover this topic extensively, you may also want to invest the $700 list price (which may be negotiable, depending on the point of purchase) for a resource that provides extensive detail for 103 sports, in some cases as far back as 1987:
National Park Visitation
You can also look at this issue from another perspective visitation to park and recreation sites. For example, trends for visitation to national parks over the past five years have been negative or flat in some cases, but on the rise for others. Among the winners have been Amistad National Recreation Area, Arches National Park, Buffalo National River, Cape Cod National Seashore, Gateway National Recreation Area, Klondike Gold Rush National Historical Park, Point Reyes National Seashore, and Yellowstone National Park.
Those with sizable drops have included the Blue Ridge Parkway, Dinosaur National Monument, Everglades National Park, Mammoth Cave National Park, Mount Rushmore National Memorial, Sequoia National Park, Shenandoah National Park, and Virgin Islands National Park.
National Park visitation statistics  (select "5 year annual report by park(s)" or one of many other menu choices).
Across the whole NPS system, visitation has been steady over the past 5 years, vacillating a little between about 273 million and 277 million. In the past 20 years, there have been a series of similar, slight vacillations, from about 256 million to about 287 million. The numbers for 2008 are nearly identical to 1992, indicating that the numbers are fairly steady, though a little lower on a per capita basis now, given the rise in population since 1992.
You can combine your investigations by both national park and certain types of outdoor activity by identifying the parks that provide a venue for the activity you're checking out.
National Park activities, by location  (select "by activity," select the activity from the list, and you get a list of parks to the right; run the cursor over a park name for a thumbnail summary, and a link to more information about the park).
Most state park systems also have data on their visitation, as do many county and city parks.
In addition, many outdoor activities have an associated organization. With a little searching, you can usually find it fairly quickly. For instance, if you want to investigate rock climbing, and find the outdoor climbing areas near you, you'd find 94,233 rock climbing routes around the world.  If you're familiar with the sport, you likely wouldn't be surprised to discover that CA has more than 13,000, but you might do a double-take when you see that even NE and KS have one each, and FL has a whopping 7. Who knows where your story will lead?
Regardless of your line of inquiry, the bottom line is that deciphering trends among different types and locations of outdoor activities may be of more service to your audience than making blanket statements.