I was interested in reading in Wired last month about the inventor of the Slinky!
In 1943, Richard James was working on instrumentation for WWII ships, when he knocked off a torsion spring from a shelf..... and it 'slinked' from shelf to books to table to floor. One patent and 300 million toys later it is as popular as ever!
Sometimes it is not about sitting down and having a brainstorming session, sometimes it is about using events that happen to come up with a new idea. It is about adapting those events to something that is commercial - thus the best tools for innovation, your eyes and your ears!