For young children
Grab your baby and sit in a dark room with a flashlight. Point the flashlight to an object, turn it on, and describe the object to your child using vivid adjectives.
Point the flashlight to an object, turn the light on, and have your child tell you something about the object (what it is, what color it is, etc.). Encourage your child to use as many words to describe the object as possible. Now switch and let him or her work the flashlight.
For older children
You'll need a blindfold for this one.
Sit in the dark room and have your blindfolded child turn the flashlight on. You describe the object one phrase at a time (It is orange. It is round. It bounces) until he or she guesses what it is. Switch roles and repeat.