I love combining acrylic paint and oil pastels.  I remember making scratch paintings when I was young using crayons and have upgraded that technique to using oil pastels.  It is a simple technique that can be applied to a wide range of subject matter.

Once the background is painted you can simply outline your subject with thick black sharpie or black paint, fill in the shape with oil pastels, paint over the shape with black craft paint and then scratch designs and patterns into the dried black paint to reveal the oil pastel underneath.  Stenciling or using markers on top in the negative space can reinforce or add to the design of the subject.