There's a lot that nature has to offer in the way of fitness. Hiking and biking might be the perennial classics, but depending on the nature in your area, chances are good you'll find a whole lot more. If there's a lake or river nearby, not only can swimming provide a good workout, your family can see if canoeing, whitewater rafting, tubing or kayaking are to their tastes as well.
Where camping is available, tasks like collecting wood, pitching tents and building campfires definitely help get families active. If it looks like the winter is going to put a damper on your workout routine, consider trading in the bikes for some cross country skies or ice skates. Kites can also make cold, windy days more enjoyable.
All in all, when it comes to family fitness the trick is to just get out there, get moving and have fun.