The apparition of these faces in the crowd

You know, I probably would have lied my ass off and said, "It's a musicological treatise" (or, in my case, "It's a plot for a new science-fiction romance novel. You can't see it until it's done."). . . you know, something your parents are definitely not interested in.

Now, of course, you'll have to come up with very interesting plans for their wedding anniversary or something, to avert suspicion completely.