I typically pack big ass salads for cold lunches. Include plenty of meat and a nice oily dressing, and add nuts or cheese if you still need to increase the calories. In terms of cooked food, some leftovers are better cold than others. BBQ chicken is delicious cold, and you can round out the meal with some cut up veggies with dip. Also, keep in mind that anything you can eat on a sandwich or in a wrap can be put in a container and eaten with a fork (or your hands, if you prefer).