Want to do something simple to help increase your dog's mental and physical health as well as your own? Its easy, take a walk.
A big back yard is great, but it is a static environment, nothing new comes into it and rarely do things change much back there. A back yard after a while is nothing more than a well planted prison for a dog. They may play frisbee or catch back there, but unless they're going out into the bigger badder world, they're not being provided a full life experience.
Dogs that walk with their owners tend to be better behaved, in better physical shape, better adjusted and have fewer behavioral issues. Owners who walk their dogs are also fitter than the non-dog walking population. Walking your dog not only allows your dog to meet his neighbors, but it allows you to meet your neighbors.
According to Cesar Millan, dogs should follow their owners to be reminded of their social status within the human 'pack', while there is little evidence to support this, no one can argue that a dog who pulls incessantly is a nightmare to walk. Whether you agree with Cesar or not on where a dog should walk, he certainly knows how to teach a dog how to walk. Whether on leash or off, dogs need to have manners or owners won't take them anywhere.
Train your puppies to walk on a leash, and by walk on a leash, that means a 6 foot leash, not a 20' flexilead. A dog should either walk on a leash or be off leash, a flexilead is not an alternative to the latter. Any puppy class can assist dog owners with dog walking tips, and any dog trainer has the tools to rehab a dog who pulls.
Flexileads are often used by people who want their dogs to feel off leash, but due to leash laws or poor training the dog cannot actually be off leash. Many of these dogs do not come when called, do not heel and because of the function of the leash, an owner has virtually no control. Cyclists on multi-use paths cannot see the string, creating possible crashes, while the string itself has caused amputations to people's fingers and dangerous attacks on dogs before owners can respond. Either walk your dog on a 6' leash or walk off leash.
If a walk is done properly there is two way communication, whether on leash or off, dogs should know how to come, heel, and "leave it". Walking a dog should not be a chore, but it should be interactive. Every activity you have with your dog should be seen as a chance to address negative behaviors and reaffirm the positive.
A walk outside has health benefits that go far beyond the simple act of walking, both for the human and the canine. The benefits are both social, psychological and physical. A six foot leash imparts a much larger benefit than does a 600 foot back yard.