You take a photo of everything you eat. The app organizes the photos by day. The daily digest view shows you everything you ate for that day.
This could be used for dieting (perhaps looking at everything you ate that day before going to sleep helps you realize how much you actually ate, as opposed to not thinking about it when you're munching bite by bite).
This could be used for personal discovery (probably everything you eat has an effect. You can start to become more vigilant of your food patterns by keeping a visual journal).
This could be used for medical purposes (need to monitor what you're eating, etc).
Anyhow, probably v1 is very simple. Version A might involve image detections. Version B might just get some mechanical turks to identify what you ate to make the visual data textual. Version C might let you make quick notes after you take the photo.
Brainstorm to where the app is something people pay to download or use.
[edited typos]