Here's an idea - team up with Subway or Togos or Quiznos or someone and offer a very simple free coach meal. A six-inch sub (nothing fancy, a couple choices like ham and cheese, roast beef, turkey, etc), a bag of chips and cookie. All of the advertising the sub chains could want, and at a decent cost to the airlines (a couple-three bucks per bag, maybe less).
Actually, UA wouldn't have to even offer them for free. Just cheaper than the current sandwich, with more stuff. Most people would pay for a sandwich combo from a major chain as BOB.