Every so often I think, "I really need to start carrying a notepad around with me all the time." I never get around to actually doing it, though.
As others have said, anything and everything is potential fodder for adventures. I have found that wandering museums and art galleries (preferably alone) generates a huge number of ideas.
Of course, like everyone else, I get my best ideas while in the shower, while trying to sleep, or while playing sports. Which would make having a notepad rather useless anyway.