Favorite smoothie ingredients

What are your absolute favorite things to add to smoothies? Lately I've been making a smoothie every day for my 1.5 year old and 3 year old because it's literally the only way I can get veggies in them. I've been adding a lot of spinach, frozen cauliflower, oats, and chia seeds, and then different fruit every day. I hate smoothies personally so I have no idea what's good to add to them. The smoothies I make always have milk, fruit, some kind of vegetable, and Greek yogurt in them.