Veggies can often be used in place of bread - for example lettuce, cabbage, or thinly sliced grilled eggplant as a sandwich wrap. Crepes made with egg batter can be filled with veggies and protein. Parmesan crisps can be used instead of croutons or as crackers with veggie toppings.
One of the closest substitutes is cloud bread. Just like it sounds, it's light and fluffy and a bit like brioche. Here's a recipe:
Favorite substitutes or recipes from others?