A good, challenging hike usually involves climbing a steep hill or mountain, and may even take you to higher elevations where the air is thinner, requiring your body to use up more energy. Therefore, it burns lots of calories.
Hiking makes it onto this list because if you're walking up an incline, you're automatically working your glutes. You're getting even more of a workout if you're carrying a backpack. And, of course, when you're on a hike, you get a chance to see some of the best scenery nature has to offer. For a 140-pound person, an hour of hiking burns about 390 calories.
Don't have any hill or mountain trails nearby? You can create the same effect you get from hiking uphill by increasing the incline of a treadmill.