The fact that a lot of kids aren't as active as experts recommend isn't big news, but a recent long-term study pinpointed exactly when couch potato-dom tends to set in: after age 9. From there, kids' exercise levels steadily drop from an average of three hours a day (amazing!) to well under an hour by the time they hit 15, according to researchers at the National Institutes of Health. So how can you work to reverse the trend -- especially this time of year, when they're apt to just hibernate? We gathered some genius ideas from parents and personal trainers:

• "Have an indoor or outdoor treasure hunt. My mom used to hide old coins around the house for us to race and find. It was very Pirates of the Caribbean! For outdoor play, put some fun, fake jewelry or stickers in plastic Tupperware 'treasure chests,' bury them in the snow all over the yard, and challenge your kids to find them as fast as they can." -- Jillian Michaels, a life coach and personal trainer with NBC's The Biggest Loser

• "Set up an indoor miniature golf course with toys or things from around the house that you can putt under, over, and around -- and use plastic bats for clubs." -- Christen Tighe, a mom of one from O'Fallon, MO

• "Bundle up, go out in the yard, and play tag, kick the can, Frisbee, touch football, kickball, or any other playground favorite. No one ever thinks about these things in the wintertime -- but unless there's tons of snow on the ground, you can still play them. Running through several inches of snow is like being on sand -- it adds resistance and really gets your heart rate up." -- Gunnar Peterson, author of The Workout and trainer to Hollywood moms like Jennifer Lopez

• "Outfit everyone with pedometers and make it a family goal to get a certain number of steps by the end of the day or week. (The usual recommendation is 10,000 steps a day.) The gadgets are inexpensive, you can get them at any sporting-goods store, and the 'how many steps have you gotten so far?' factor keeps everyone motivated." -- Catherine Holecko, a mom of two from Neenah, WI