My husband and I did a trip without spending any additional money in the parks (other than on a couple of alcoholic beverages at Epcot).
We had the DDP though, not the QSDP. I don't think that would make a difference though in our spending habits...it would still be more than enough food. We usually would get one soda/milk to share, and get a bottle of water with the other meal so we would have something to drink around the parks. By the 3rd day or so, we both started getting bottled water because we were sick of soda and other beverages. lol We also used our resort mugs at our resort a lot! I loved getting hot chocolate in the mornings, and we would be back at our resort by 7pm or so at night and would fill up with soda if we felt like it.
We did turn down a lot of desserts at table service restaurants...it's just too much food. At QS restaurants, we would get portable desserts like a cookie to eat later on, or ask for grapes or some other fruit. We saved all of our snack credits and used them for breakfast (like a huge yogurt parfait at our resort...CSR, or a large muffin, or a cinnamon roll).