Teens learn by example and whether they care to admit it or not, they typically enjoy doing things as part of the family. Go rock wall climbing or mountain climbing. Go bike riding as a family. Camp in the great out doors and go hiking, boating, or biking yet again. Learn a new sport together. It's amazing how much fun you can have learning how scuba dive and while you are at it, neither of you will be consuming empty calories. Even if the sport you take up together doesn't involve a lot of physical activity it is quite likely more active than sitting in front of the television.
To help with your teenagers obesity, have them join a recreational sports team. We have all kinds of sports teams available in our community in which our teens can join. Even those with no skills at all can join and play in some of the leagues while other leagues are competitive. Getting out and about for a softball game, soccer game, and even volleyball can be a great way for the family to do something together and the games and practices are opportunities for your teen to be active.
Lawn work is another way to get out and get active with your teen. The key in your teens obesity, as always, is in your teen burning more calories than he or she consumers. It is often easier said than done but it is quite possible if you work to get them out and about. Find things however that will be interesting to your teen and focus on those rather than torturing them with activities that hold no interest at all (well maybe not the yard work). At any rate, filling their time with activities is also allowing time that they aren't consuming calories as well and that is something worth considering.
