I quite literally have a folder called 'concepts' where I toss notes with ideas I have but don't have the time, wherewithal, etc. to develop at the time. Sometimes I come back to them. Sometimes I delete them because they were stupid. A few get promoted to WIP.

Samples:

• The Fun-stage Situation: A guy shows up, captures some people, maybe a family, and forces them to do fun things under penalty of death.
• Where's Waldo and Carmen Sandiego travel the world committing crimes
• Lifecycle: Two ships crash into each other, they merge materially and as crew, and grow by devouring other objects but it eventually grows unmaintainable and 'dies.'