It's a joke. A blog about grapes. Grape art. Grape poems. Grape music. And he's kind of embarrassed that the teacher encourages him, because he was just being sarcastic when he said it was a grape idea.
But it is sort of thrilling to imagine until he decides it's stupid. He thinks about it, but he never does anything about it. He's afraid he'll run out of things to say about grapes. He's afraid that people will think it's dumb. Eventually, he stops thinking about it. His life unfolds in a common way.
It is sort of thrilling to imagine. After eating all the grapes off the vine, he likes the way the vine looks like a little tree so he takes a picture of it and that night starts a blog, "It's a Grape Idea". He vows to post something about grapes everyday for a year. His life unfolds in a most unique way.