If you are designing your yard with the family in mind, then something like a playhouse or even a tree house are great additions. Not only are they fun for the kids, but they also round out your backyard landscape in a big way. Suddenly, with the addition of these elements your yard becomes more than it was. It is more functional. Now, if you do not have a family you can still design your backyard landscape to have social functionality to it. You could consider an outdoor kitchen or a covered patio with a big table for larger dinner parties. Either way, it all looks and feels great.