A banana and curd base is the most reliably accepted combination. Banana provides natural sweetness, potassium, and binding. No added sugar is needed if the banana is ripe. Full-fat curd provides calcium and protein. Adding a handful of spinach, which is tasteless when combined with banana, is a widely used technique for adding iron without detection.
Other combinations: mango with coconut milk and cardamom, strawberry and banana with curd and nut butter for protein, or chikoo with milk and almond. Avoid adding refined sugar. A tablespoon of ground flaxseed provides omega-3 without affecting flavour. Serve immediately after blending to preserve vitamin C.