Here are some ideas. . .
Disguise exercise as a game. “The ground is lava,” “who can do the most cartwheels,” etc. Have fun with it!
“Backyard Olympics” with tinfoil medals. You can inspire your kids with great feats of strength: push ups with a kid on your back, hikes with a kid riding piggy back, whatever. Heck, you can even do pull ups while your kid is on the swingset, or teach your kids to do handstands!
Go fly kites. The less wind the better because the only way they can get their kite to fly is to RUN! They won’t even realize they are wearing themselves out.
Try geo-caching. Kids of all ages love it! (It's a treasure hunting game where you use a GPS to hide and seek containers with "treasures" inside).
Got a stubborn teenager who doesn’t care about exercising? Find a good hiking spot, like a waterfall, cave, scenic overlook, ect. Even teenagers love the outdoors – so do things outdoors where they can get moving and get their heart rate up in a way that's fun. Exploring, climbing, swimming…these are all good options.
One final thought: you’re a superhero to your family whether you realize it or not. You are your kids’ biggest hero; how you act will determine the health and habits your family builds too.
They will do what you do, not what you say to do.
If we can instill the habit of health, fitness, and happiness in ourselves, our families are more likely to grow up healthy and not deal with the health issues that come with being overweight and out of shape.