In my opinion, a well-made sandwich should have a little bit of all the major food groups. Generally, you end up with carbs, veggies, and protein, but don't forget that fruit is also an option for a healthy sandwich topping.
Mix it up. Bring new tastes and textures to your sammy by adding some fruit between your slices of bread. Sliced apple, pear, and grapes work really well, but I've also tried mandarin oranges, bananas, kiwi, and mango. Not only does the fruit add a hint of sweetness and distinct texture to a standard sandwich, but it also adds some always-needed extra fiber to your diet. This works with my girls, too. They love apple slices on their grilled cheese as much as I do.