From the outer of New York to the heart of Singapore, nature is everywhere you're not looking in some of Earth's most crowded big cities. Outdoor adventure is often just a few stops on the train - or a short drive- away from the annoying crowds.  Here are a few favorites to get you going: Mount Hollywood Trail, Los Angeles It's not a skyscraper or a Ferris wheel, and there's no expensive restaurant up top. But if you want an absolutely unforgettable, no-admission-charged, 360-degree view of the entire Los Angeles region , it's yours. All you have to do is walk a little. One of the preferred exercise routes, the Mount Hollywood Trail, is easily accessed from the parking lot of the famous Griffith Observatory; you see what you get at the end of this steep 1.5-mile trail, which winds its way through the Griffith Park wilds up to the mountain's 1,640-foot peak. Bronx River, New York It may not be on your Top 10 list of things to do when you finally get to the Big Apple, but exploring the almost-secret river that flows through one of New York's most mysterious boroughs   is an unforgettable experience. The Bronx River Alliance, a non-profit group that has worked tirelessly to bring the much-abused river back to life , operates cycling and canoeing trips along the river, from the rugged Hunts Point section down near the harbor on up through the borough. Southern Islands, Singapore Believe it or not, tightly packed Singapore does have plenty of open space left . There are actual trails right in the middle of everything, if you know where to look . But to really get away from it all, the Southern Islands, a small group of little green dots just off the city-state's southern shoreline, are just the thing. Lee Valley, London This East London river valley changed last summer, when anyone near a television got an eyeful of Olympic Park, built directly in the middle of the valley. Inside the park, beautifully landscaped sections quickly became a favorite relaxation spot for games goers. At present, you'll find miles of walking paths worth exploring. Start at the Waltham Cross train and follow the signposted walking route southbound, guiding you past important historic industrial areas, through interesting city neighborhoods, past the western side of Olympic Park and on to the Thames. If someone wants to enjoy an unforgettable experience for free , he can go to   _  .
Mount Hollywood Trail and Bronx River Mount Hollywood Trail and Southern Islands Lee Valley and Southern Islands Bronx River and Lee Valley