Sean Penn and Shannon Costello at Nobu in Malibu
09/14/2011 AT 10:00 AM EDT
and girlfriend Shannon Costello
shared burgers and soda at The Counter in Santa Monica, Calif. The actor was wearing a suit and they were "affectionate towards each other," an onlooker says of their lunch date. Adds the onlooker: "[They were] constantly smiling during conversation."