Jazz up plain mayo by stirring in orange marmalade and chopped fresh rosemary, then seasoning with salt and cracked pepper. Some more ideas: Chopped chipotle peppers and sauce, other fruit preserves, or mashed avocado and ground cumin.
Hearty fillings are best between slices of whole grain or artisan bread—it will keep your lunch from getting soggy and will hold up longer in picnic baskets. For the best results, place wrapped sandwiches on top of ice packs in a cooler—they'll stay chilled and they won't get crushed.
Just about any type of meat will work, just make sure it's sliced. Seasonal vegetables—when also sliced ubër thin—stack nicely and add flavor, crunch, and nutrition.