Almost anything which is not directly related to the problem at hand.
I browse books, shops, and real-world venues, listen to music and podcasts, watch films, visit art galleries and museums, play with my kids, engage in conversations, travel—anything which might spark a new connection.
In an interview with Wired, Steve Jobs said, “Creativity is just connecting things. The broader one’s understanding of the human experience, the better design we will have.” (http://archive.wired.com/wired/archive/4.02/jobs_pr.html)
Exploration, play, and insatiable curiosity. To me, these are key to creativity.