Take a look at some of the amazing art at this year's Burning Man festival...even more impressive when you realize it's all created to exist for one week only. When the festival is over, the desert landscape is returned to the natural state and there is no sign that these things ever existed.
It reminds me of the story of an artist who wanted to explore new directions. Every day for a month he created something and at the end of the day destroyed it. This freed him from worrying about making judgments about the work as it evolved and to experiment more than he normally might have.
Also, if you need inspiration for a sci-fi or fantasy story, these pictures should do the job.