I looked up play in a dictionary. One of the meanings of play is "recreational activity, esp: the spontaneous activity of children". Another is "to engage in or occupy oneself with <play basketball>" and another is "to engage in (an activity) as a game". One definition of recreation is "a means of refreshment or diversion". Those definitions do not state anything about the nature of such play, and do not mention skills needed. Other definitions of play do, but those refer to different usages than the one used to refer to a child amusing themselves. I would define play as an activity engaged in for the enjoyment of doing it, without regard for any results.
When an autistic child flicks xyr fingers repeatedly in xyr face, for example, is that play? Depends on the purpose of that behaviour. If the purpose is, as it often is, to occupy, amuse, or refresh themselves, if it is done for the enjoyment of doing it and not for some result the behaviour may have(such as getting attention), then it is play, regardless of whether most people would find it amusing to do.
As for imagination, it is defined as "the act or power of forming a mental image of something not present to the senses or never before wholly perceived in reality". Creative is defined as "marked by the ability or power to create: given to creating <the creative impulse>". Temple Grandin says about her visual thinking: "I create new images all the time by taking many little parts of images I have in the video library in my imagination and piecing them together." Obviously she is imaginative. Although not all autistics think like her, many think similarly. Personally, I think the definition of imagination should be changed to the act or power of forming a mental representation of something never before wholly perceived in reality to recognise that imagination need not be visual. Many composers have what I would call auditory imagination, the ability to imagine hearing a whole symphony they are composing. Most people would consider someone like that imaginative.
The only reason, it seems, that autistics are not considered imaginative is that some people think imagination means making up stories with toys as a child. That is certainly imaginative, but isn't someone who mentally takes bits and pieces of things, puts them together, and watches the result work, or someone who mentally listens to a full symphony play in their head without ever having heard it in real life, imaginative? Autistic people often show their imagination less obviously, but that does not mean it isn't there.