When children play we often just think they're having fun, but research has shown that the benefits of play extend far beyond that in early childhood. A child's brain doubles in size in the first year of life and by the age of 3 it has reached 80% of its adult size. This means that what your child is exposed to in the first 3 years will have a massive impact on their future.
Very often just letting the child play as they wish is the best approach, as you don't want to interfere with their progress, but there are certain things you can do to help them along. For example, allowing a child to engage in messy play has a place in that and it can help them to discover the world. You should also try to introduce them to scribbling and finger painting as young children are natural artists.