Whilst I'm no expert, it seems to me that the two most important things that are helping to develop Libby's imagination are reading books and playing without adult intervention. I seem to remember reading that allowing children to dictate their own play rather than interfering is one of the best ways to teach them to 'think outside the box'. Whilst I'm not keen on that term, I love the concept. I like the idea that Libby will grow up to be independent, free thinking and not constrained by societal boundaries.
As for books, I can already see the advantages that Libby is gaining from them. We haven't attempted to teach her to read yet, but when shown text she already looks at it from left to right, clearly copying what she has seen us do when reading to her. Today when she saw me writing, she commented "L, L for Libby".