That's a difficult question. I'd work around it by at first adjusting her meals. I would make her home-made junk food (let's take hamburgers and fries for example), and start adding fresh vegetables to it. After a short while, I'd replace the beef patty with a chicken patty (and end up replacing it with a soy patty in the end) while also reducing the fries portion.
The gist of it is that she musn't be able to tell the difference between the taste of her meals. That's why you need to slowly introduce substitutes and healthier nom noms into her current diet.
I'd also suggest buying fresh fruit (different kinds), wash it for her and place it next to her when she's reading or doing something. That way she'll figure out which fruit she really likes and eat that instead of crisps, cookies, whatnot as a filler.
Another thing concerns drinks - if she drinks a lot, she's probably drinking beverages like 7up, or Gatorade, or whatnot. If she doesn't like plain water, just mix water with her favourite juice (or tea). At first, most of the glass should consist of juice with a tiny bit of water (make sure that the juice has a high percentage of fruit content), and then just slowly decrease the quantity of juice and add more water.
Introduce different kinds of salads and dressings as a side-dish during the main course. Use red and green lettuce, tomatoes, beans, bits of cooked potatoes, light yoghurt dressings, vinegar, pumpkin oil, olive oil. Anything you can think of. She's bound to enjoy something.
In general, try to eat out as little as possible (especially take-aways), just about any food you order over the phone / internet is quite fatty, because they use lower-quality ingredients and oils.
P.S. Olive oil. For everything.
Hope I managed to give you some tips or guidelines on how to approach the issue. Telling your child she is fat and that she needs to lose weight doesn't usually help at all. If she likes it, she's going to keep doing it, so you need to subtly change the meals she's consuming.
As far as activities go, I'd suggest trying to take her for a 30 minute stroll (or more, if she's up to it) every day (or every other day). Slow-paced walking is just as good for her.