I'm sorry, but I think the double burger, extra bun trick is....well, I just can't bring myself to do it...Now I might order a double burger and split it with someone that's not nearly so bad....I can do without the extra white bread anyhow.
Fruit is a fairly reasonable snack, and an apple can be quite filling. There are a number of places to get apples on property.
We share snacks like pretzels and popcorn....but hands off my Mickey Bar - get your own!
Norway's bakery has a meal deal of some kind. It sounds a little ordinary foodwise, but looked to be a good deal. On the Boardwalk you can get a slice of pizza. (corndogs too, I think)
A cup of chili is enough for a small meal. It's a good value for the price. There are several places at WDW to get some. In general, checking out the soups is often worthwhile. Saltines aren't the best, but they may come in handy afterwards.
At QS we often try to order one less meal than we think we want - that way we are sure to share, and eat everything. But we often go to WDW as a family group. It's hard to do leftovers, but if you are dining at your resort's food court and are headed to the room to get swimsuits...it might be worth bringing those leftovers back to the room. With kid's meals - the applesauce is easy to save - just don't store it in a fanny pack with your valuables!
Eat a buffet meal either as a late breakfast (brunch), late lunch/early dinner (linner). That way you can eat two meals on that day - or if you do a regualr dinner time - keep that in mind earlier in the day. The morning after a dinner buffet, we usually only eat a small breakfast. In this way, buffets are actually not a bad deal, since they inlcude a drink for the price. Breakfast is like $18. So if you eat late - that's like a $9 meal allowance for breakfast and lunch. The orange juice at CP is really good too - and you can get oj and coffee refills.
Having a few breakfast/snack items in you room will save a few bucks. If you can't/won't bring items from home, and you don't have the means to hit a grocery store; you can at least scan the gift shop of your resort. They may not be the cheapest prices, but they have a few items there for less than food court prices. But a few things from home are a good idea - like a bag of goldfish crackers or granola bars.
Bring Bottled Water! Or buy one bottle and refill it.