You can eat, read books, drink wine or beer, play cards or board games, do pretty much any quiet, low-to-the-ground activity, using the music as an excuse to be there. Listening to the music is an option (about 75% do listen), but not a requirement.
If you try that in New York City, keep an eye on your shoes. (The story at that link reminds me of the shoe revenge taken in this book.)
UPDATE: And you're not even supposed to talk at a rock concert, according to the Washington Post (link via Throwing Things). Jeez! The last time I was at a rock concert, you had to shout right in the ear of the person next to you to be heard and no one else would even notice. What has happened?