Times Are Changing but You Can Still Enjoy Nature.
I'm probably just getting old, but it seems to me like kids used to spend a lot more time outside. Before a million channels on the television, expensive video-game systems and expansive Internet...we had to create our own fun. Nearly all of the indoor entertainment opportunities are completely passive, while outdoor fun is interactive and creative.
One side-effect of this trend towards passive, indoor activity is (in my opinion) a reduction in thinking abilities. Back when you had to be creative in coming up with games and make use of the tools Mother Nature provided, your brain was continually stimulated. That doesn't happen while you sit on the couch, watching the latest reality television show. Another side-effect of this change in behavior is the obesity epidemic our nation is currently entering. Typically, outdoor activity is active activity.
If you would like to encourage your kids to spend more time outside you can certainly try to do so by telling them to or asking them to. Of course, most of us remember how well that sort of thing worked on us as kids. Some parents actually use the concept of spending time outside as a punishment which, as you can imagine, isn't the best course of action. Kids should associate positive things with spending time outside, not negative things.
Planning a family camping trip is one of the best ways to introduce kids to wonders of the wild outdoors in an effective and, hopefully, permanent way. Grab a tent, some sleeping bags, some hot dogs and marshmallows and go camp somewhere fun. The goal in doing so is to show your kids how much fun the outdoors can be and let them decide, on their own, that they like spending time there.
After all, kids learn better by observation than they do by simply listening. If you want to get your kid of the couch and out into the world then perhaps you need to get off of the couch and out into the world? Show them how much fun being outdoors can be, teach them the games that you used to play when you were a kid. If you can't remember, do an online search for "camping games" and you'll come across more than enough information to job your memory.
While it's not a surefire fix-it-all, regular camping trips will go a long ways in helping you fight the couch potato syndrome with your kids. Give it a shot, make it fun for both you and your children.