It was an ICEE button.
Unfortunately the car didn't actually make me an ICEE. I just pressed the button and a guy came over the speaker and said "If you just drive up the hill, we'll have a vanilla ICEE ready and waiting for you."
Now, I realize vanilla ICEEs don't exist, and if they did... that would probably be pretty gross. I'd settle for a cherry.
If anyone is interested in helping me pitch an ICEE button for cars, let me know.