Make sure to take several servings of fresh fruit with you to work, as well as your regular lunch. It makes a huge difference in the energy level, which is really important with a new job.
My vote is for leftovers from the previous night's dinner. I've known a number of people who take their dinner leftovers for lunch in a microwaveable pie plate (covered well with plastic-wrap to prevent leaks)
Sausage and cheese with crackers always makes a nice change-of-pace lunch.