I know this won't be feasible for people living in suburbs, but for the ones who don't I thought I would share.
I have a 5 yr old son who likes to hit the trails. He is very active around the house, however in school these days kids don't get very much physical activities. Also, we do not have the time to do overnights or day hikes every weekend.
So, to keep his legs in shape and to test his endurance/mileage we do walks on the rural roads close to our house. This may not sound fun to many, however there is alot for a young kid to learn from this, at least here in the South. There is alot of people here who are not environmentally aware and altogether lazy, however they love beer. So this in turn leaves ALOT of aluminum cans on the side of the road. Walking the roads here, we find about 35 cans per mile, if we haven't walked it in a couple of weeks. We get 60 cents per pound right now when we recycle them at the salvage yard. That equals about $1.20 for every $1.25 for every 2 miles we walk considering a 12oz can weighs .5oz(tall boys/16oz cans weigh a little more).
Finding the cans is like an adventure for my son. He also love crushing them with his feet. He get to have fun, while we both get exercise. The time together is priceless. The teaching time is also equal. I get to teach him about trees, wildflowers, animals(alot of dead ones), how not to litter, how to be resourceful, how to recycle, how to stay hydrated etc. He also learns the value of a dollar, because he keeps the money and saves it.
It may not be as good as day hikes in the mountains, but its a little easier walking and rewarding for both you and the child.
Try it.....you'll see how much fun it is.