If you put peanut butter on both sides of the bread it will help keep the bread from getting soggy. Otherwise, pack everything except for sandwich in advance.
I agree with other PP's. Usually pack a fruit (clementine, banana, cut strawberries, blueberries, mango, peaches, etc).
I include veggies with main part of lunch and/or add sliced cucumber, red pepper, tomatoes with a little bit of feta, carrot sticks, etc.
For protein/main course, I do a leftovers in a Thermos (warm up and put in Thermos in the morning), sandwich, sandwich rollups (tortilla with lunch meat, cheese, sometimes add a mustard, etc and either kept whole or sliced into bite sized pieces). Or I do a more snack-oriented lunch with cheese/crackers, pretzels and hummus, cheese stick, yogurt etc.