When you play your harp around a group of butterflies in specific areas of the game, a Goddess Wall sometimes appears nearby. Hold your sword up to the light, then draw various symbols on the wall in order to obtain the related item. For a group of recovery hearts, draw a heart. For a bunch of rupees, draw a rupee. Drawing an arrow grants you arrows. Painting a circle gives you bombs. And if you can manage to draw the Triforce, you'll get some fairies.