Someone mentioned an Aussie cow dog. I have a 3 year old and she is an amazing dog BUT, you need to know that the breed is a very much one person dog. She is my shadow. She will get up and folow me if I take 3 steps. I trip over her 20 times a day. She is way too smart for MY own good. Aussies definately need a ton of activity because they are bred to be very high energy / endurance animals. Just roaming free around the yard will not do it. They crave dog / owner interaction and will do anything to please "Master" but to leave one alone is asking for trouble. They will find ways to entertain themselves in your absence. Usually not "good" ways either.
If you can handle that, they are incredibly loving and loyal dogs, tough as nails for their size and afraid of nothing. My female is 35 lbs of pure muscle. Lightening fast and amazing to watch work other dogs as cattle. She wil heard anything that moves, kids, dogs, cows, birds, it doesn't matter. They tend to have a huge small prey drive so critters, cats and the like need to beware. It takes a firm hand on the leash if a squirrel or chipmunk crosses your path.