Hmmm, typically I recommend nut butters (like peanut butter 
), fruit, or milk/oat milk, but not wanting to add calories is valid. You could always try adding a little less powder, especially if you’re adding something with protein like peanut butter.
Aside from that, I think cinnamon, pumpkin spice powder, or cocoa powder/Nesquik could be a good addition! You could create a Mexican Hot Chocolate situation with cinnamon, a little chili powder, vanilla extract, etc.