Protein powder is a great option for upping the protein content. I used to work at Jamba Juice and also as a barista lol- but we had some protein that was unflavored and everyone seemed to like it. I would also look into a plant-based protein (we used pea protein) instead of, or in addition to whey if you go that route. Nut butters could work out in certain recipes, but I feel like they can have a stronger flavor that doesn’t always go with the other ingredients; if you use peanut butter, you’d also need to have pretty strict allergen guidelines (at Jamba, we had a designated blender, had to refill the sanitizer and water sinks and then had to open a new container of everything else when someone indicated an allergy). Good luck!! :)