While there are few opportunities to save on a one day pass to WDW, there are a few ways to save money in order to make the day more affordable.
1. Eat a large, early breakfast off-property. If you can fill your stomachs early, there will be less need to eat during the first half of the day. Egg McMuffins come to mind, but you catch my drift.
2. Bring your own meals. Determine that you don't need to eat at a table service restaurant while enjoying Disney's other attractions instead. Subs from Publix, items that some people eat at ambient temperature such as fried chicken; whatever your particular culinary habits will allow. Bring it all in a container of choice, and eat at your leisure. The Tommorowland Terrace eatery has outdoor seating and can accommodate anyone eating their non-Disney food.
3. Bring your own snacks. Trail mix, cookies, and candies can all be stored in sandwich bags for the sake of individual portioning.
4. Save the souvenirs for another trip. Why make the plastic industry in China any wealthier on a trip where you're just trying to have a little fun for the day?