My kids are the same, especially my daughter. Doesn't eat much, but is always up for a "mana milk" (her word for banana milk). I use smoothies as a vehicle to smuggle a whole lot of nutrition into her diet.

So, as you might be able to guess, I use bananas & milk as my basis. Makes it nice and sweet and thick.

I also sometimes add frozen berries (or fresh, when strawberries are in season or I need to use up some blueberries). DD insisted on "pink" milkshake for a while (everything had to be pink) so I added strawberries to sooo many things.

I almost always add some flax seed oil for the omega3. But not too much, so you can't really taste it.

I often use half milk and half kefir (a cultured milk that I make myself) for the probiotics. You could use plain yoghurt or buttermilk instead.

I also sometimes add honey, if it needs sweetening.

Or bee pollen (which is meant to be a super food). Either sprinkled on top or blended up with it.

I have tried almond meal and rolled oats, but none of us likes the grainy texture. But it would turn it into a liquid breakfast.

I also tried smuggling in some cod liver oil with mixed results. I wouldn't recommend it, but she did drink it.

I would suggest starting pretty plain first and then, once your child is "hooked", you can get a bit more adventurous.

ETA: I also make icypoles out of some of the thicker smoothies.