I usually eat part of my lunch around 10:00am since I eat breakfast at 6:00am. If you find yourself getting hungry throughout the morning, I suggest eating a light snack, something you can eat quickly and on the go! I also drink either coffee or tea in the morning and try to drink at least one bottle of water in the afternoon.
Another important point is that I work at a school, so I usually get home around 4pm and eat a light snack before dinner. If you are at an office until 4 or 5pm, you might want to pack a little extra.
Apple, Dannon vanilla yogurt mixed with Grape Nuts and walnuts, Luna bar
PPJ on whole wheat bread, banana, Clif bar
Yogurt with Grape Guts, oats and walnuts, carrots, homemade banana bread
Salad with carrots, raisins, almonds and cheese, balsamic vinegar and olive oil for dressing, banana, Luna bar
Leftover black bean and corn salad, granola bar
When I pack a lunch, I try to include at least 1 item that is fresh, whether it's a fruit or vegetable. That way I'm not eating all processed foods. I make sure the foods that are processed do not include high fructose corn syrup or trans fats (after my big realization a few weeks ago).
I hope this gives you a good idea about what a personal trainer would suggest eating for lunch. If you are packing your lunch, you are already ahead of the game because that means you are spending time with your food and you know what you are putting in your mouth! Good job!