Sure I am into all the outdoorsy stuff, but when it comes to kids activities, I honestly believe that hiking is one of the best. Besides being something to get them outside doing something that isn't dependent on wifi, there are lots of other great attributes:
1) Ages 2 Plus. If they are steady on their feet, they can hike. First it might be some short nature trails and then you can work your way up slowly. In the toddler years it was walking around on farmland. When Brody was four years old he could do 2 to 3 solid miles at Ijam's Nature Center in Knoxville with not much elevation gain. When he was five years old he could walk about every trail in Pinson Mounds State Park over the course of an afternoon. On his first hike in the Smokies (six years old) along the Appalachian Trail, he did around 6 miles with about 1,200 feet in elevation.
AT White Blaze!!!
2) Super Safe. There are no vehicles, which means you can pretty much let them loose. Even when you walk on greenways, you have to worry about bicycles. On trails, they can run wide open and you don't have to worry about collisions.
Just Keep 'em in Between the Adults
3) Low Barrier to Entry. It you possess shoes, you are already well outfitted. Throw a small day pack on to carry some snacks and water and you can stay out as long as you could possibly want to.
4) Super Cheap. Every parent has had the experience of spending some ridiculous amount of money to go somewhere or do something that has been a total bust. Because hiking essentially is free, it is all upside. Even if the outing is a total disaster, at worst you took a short road trip to a park or trail.
So remember, pack something warm to wear and plenty of snacks and drinks to eliminate potential for whining. Actually, you probably won't encounter much whining. I have been in the woods with three and four years olds that whined significantly less than most adults I have taken on trips. It is always embarrassing when you see a 30 year old ask to be carried after they run out of chocolate chip cookies.