This book of bodyweight exercises has been my workout bible for years, but I also love hiking, or as the cool kids call it, rucking. Rucking is hiking, with some extra weight on your back. You don't need any special gear to ruck; just a backpack and some sand will get you going. But put 20 pounds in your average day pack, and you'll quickly discover it's not very well made. When you wear through what you've got, get a GoRuck.