Mash avocado in a small bowl with lime juice. Season with salt.
Spread the mixture evenly over each rice cake.
To make the witches, place 3 or 4 matchsticks of carrot on either side of 8 rice cakes to form the hair.
Cut 8 small triangles from the nori sheet, using a rice cake as a size guide, then place triangles on top of each rice cake to make witch’s hats.
Place 2 sesame seeds below the hat to make the eyes.
Place one matchstick of carrot in the center of the avocado mixture, facing upwards to form the nose.
To make the one-eyed monsters, arrange 4 or 5 matchsticks of carrot on top of the remaining 8 rice cakes to form the hair.
Place 1 slice of string cheese in the center of each rice cake, then place 1 caper on top of each. Serve.