Add any of the following to your burger:

Fresh Avocado
Portobello Mushroom
Bacon (two slices)
Beef Patty
Pineapple (two slices)
Jalapeno
Onion Rings (two rings) 
Cheese – Cheddar, Swiss, Pepper Jack, Blue Cheese 

Substitute beef with any of the following on your burger:

Chipotle Black Bean Patty 
Salmon Fillet
Chicken Breast
Masala Chicken
Gluten Free Bun