I'm with Josin on doing another writing prompt. That contest (teen diary) was definitely the most fun. Specific ideas might be: a letter from OR to your long lost sea captain dad (inspired by my current reading of Pippi Longstocking to my daughter); a description of a haunted house (inspired by House of Leaves. Sorta); write about being in an amazing moment (or seeing something amazing) while also describing a memory IN CHARACTER (e.g. 17 year old Greg is riding on a roller coaster but remembering the time his older brother accidentally killed his pet bird--but, you know, better and cooler) (inspired by Hollis Frampton's very awesome experimental short film Nostalgia).
Another idea, we could pitch for a book that's not ours and is already published.