If you are looking for a smoothie, I would go with the base of frozen banana, spinach and almond milk. After that, you can add peanut butter or other fruits to add protein or alter the taste. It will give you a rich taste and keep you full.
If you are looking for something a little thinner to get it down between contractions, you could thin the above recipe with coconut milk or try juicing. You could juice kale, ginger, carrots, apples, and pineapple (a fave of mine). That will get you lots of vitamins and I think it helps keep my full longer. Also, if you end up sick during labor, it won't be miserable to get back up (plus the ginger should help nausea!).
Coconut water, honey, orange, berries, kale and a carrot. I make it to taste, but I usually put a handful of Kale in it. It's not a strong taste IMO, so it's my favourite to mix in. Coconut water is amazing for energy