I’m new to this, so someone else should chime in as to what exactly is or is not allowed on the diet.
I think the key to a good green smoothie is A) something a bit sweet, and B) something a bit creamy. Normally I would use bananas for creaminess and sweetness. Obviously that’s not alloweed.
I would play around with a combination of either avocado or coconut oil (for creaminess) and a tiny bit of stevia (for sweetness).
You could add herbs, like mint, for extra flavor. Adding lemon might give it some tartness as well, and citric acid can help break down the spinach so your body can utilize more of its nutrients.