I'm a Lifetime Member of WW and I am always nervous about gaining weight on vacation. The walking at WDW helps, but I'm a person who does intense exercise 5-6 days a week, which includes intense cardio and weightlifting. The walking I do at WDW (with 2 small children) is definitely not getting my heart rate up enough to consider it cardio. But I am neurotic and I will exercise on vacation (I've done it many times before). That said, other things you can do:
1. Try and stick with the filling foods as much as you can. Even if you are not formally measuring food and counting points, these foods will satisfy you faster, you will feel better physically (they're not rich foods in the heat). Stick to fruits as snacks, salads with chicken for lunch, etc.
2. At buffet lunches or dinners, fill up on veggies first. Choose lean sources of protein and limit fried foods and cream sauces and soups.
3. Drink lots of water...you will need to in order to avoid dehydration.
4. Don't eat until you are completely stuffed. You should feel satisfied, not hungry, yet comfortable.
5. Allow yourself some treats here and there. If you don't, you may binge later on something you don't even enjoy as much anyway. Have a little, get it out of your system, move on.
6. Make sure not to obsess about food, and enjoy Disney World for ALL it has to offer. Take in all the attractions, the sites, the beauty of each theme park, enjoy the people you are with and your resort too!
Sticking to the above 6 things should help you to avoid any major weight gain, you should feel well physically while doing this, and when you get home it will be much easier to start counting your points again daily!