Play is about creating, imagining, sharing ... and anyone can do it! The ability to "invent" is one that is being pushed aside, as we find ourselves in toy aisles filled with every kind of already-made play props. There is a place for all these toys, but a need for imaginative approaches to play, as well :)
We have to make sure that Invention, Problem-Solving, Creating - all great skills we look for in adults - are encouraged when children are developing!! Open-ended projects, where materials are gathered and new things are made, are great for helping to develop these skills. Start with one project, where everything is created from materials you already have and see the wheels start to turn. You might have to model and vocalize the thought process for making playthings, but the fun will catch on!