Have a chat with them and see if there's any physical activity they'd really enjoy, something like dancing, martial arts, horseriding or joining a team such as netball, basketball or football or even something like RAF/Army cadets, water sports and outdoor pursuits - there's so much out there. I know where I grew up the local authority ran taster sessions in the school holidays.
If you bought them a bike would they go out cycling?
It will make getting out and active fun and enjoyable and may lead to a healthier alternative in general.
Why not get the family involved, go out for walks or bikerides, go swimming together, find out about charity fun runs or swimathons and train together to raise money for charity, maybe something close to home they can identify with.
Have you or anyone else you know have a dog they could take some responsibility in walking?
Set them up with a good healthy breakfast so they won't get hungry early and be tempted to snack on un healthy stuff or give them healthy snacks to take out with them so they may tuck into them in they're hungry rather than going to get crisps and chocolate and such.
Try and make healthier alternatives to their favourites such as fresh fish and lots of fresh peas and such with low fat oven chips. Make little changes over time like using frylite instead of oil, buy a george forman gril for meats and such, changing to skimmed milk and cutting down sugar in their drinks gradually.
Introduce more fruit and veg gradually, cook fresh rather than processed food esp microwave meals, and cut way down on any fizzy drinks and such. Don't have unhealthy snacks in the house but have lots of healthier alternatives available such as fresh fruit juices and fruit.
Maybe buy a blender and have some fun making some different fruit smoothies and healthy soups in them, get a feel for what they like. Maybe get