Provide extra materials for your child to work with. For example, if your child is working with play dough, provide her with some cookie cutters and plastic knives. Also, consider adding more challenging items to the play such as a garlic press, blunt scissors and toothpicks. Observe how your child uses these materials and add more as needed. Ask your child for suggestions as to how each of these materials can be used with the play dough.
Make suggestions to your child as she is playing. For example, if your child has found an empty cardboard box and is crawling in and out of it, ask her if she would like to pretend she is a circus animal on the train on her way to the next city. Explain to her that animals in the circus travel by train to each of the cities the circus visits. Find some pictures in books or on the Internet that show animals traveling by train. Ask her if she would like to decorate her box like a circus animal cage. Brainstorm together the types of things an animal on a long train ride would need and decorate the box accordingly. Bring out more boxes if she would like to add more animals to her train.
Ask open ended questions of your child as you play together. For example, if he is building with unit blocks, ask him what he is building. Tell him that you would like to be his contractor and that means he is the boss. As his contractor, he gets to tell you how he would like his building to look, as well as the types of things that will happen in his building. Keep asking questions as the play progresses. Add more materials, such as small cars or plastic animals to keep your child actively engaged in his chosen activity.