Rattles and crinkly toys are pretty straightforward toys that easily demonstrate the concept of cause and effect.  Your baby won't understand the idea yet but with time, they'll be able to associate that their actions can cause a certain reaction or effect. For eg. when they shake the rattle, it makes a noise. It's also uncomplicated and appropriate for their age. A pounding and musical toy is also quite an effortless way of teaching cause and effect for toddlers. When they hit the toy, it rolls or makes a noise.