The New York Times has an interactive map of the best ballpark food around the country. We visited their recommendation, Leo's Coney Island, on our recent trip to Comerica Park and weren't disappointed.
Kurt went with the NYT approved traditional gyros while I tried the chicken gyros. Both were excellent with high quality meat, fresh veggies, soft pita bread, and tangy tzatzki. The portions were large, and at $6.25, it is one of the best deals in a venue where hot dogs and slices of Little Caesar's pizza sell for $5 apiece. There aren't many vegetarian options in the park (unless you are ok with nachos for dinner), so the vegetarian pita was a welcome sight on the menu.
Be sure to bring your ticket along. They have a street entrance as well as a park entrance, and you will need your ticket to return to the game. Leo's is located on the outer perimeter of ground level just past first base. It's small and tucked back in a corner but definitely worth tracking down.