I would say not great ideas, but small ones. One's I'd like to revisit in 6 months' time and mull over, connect the dots, etc.

I've been keeping a "sparks" log (can't remember where I read this, somewhere on HN) where I just jot down small ideas for side projects, etc. when I have them.

Then when I have some free time, I pick an idea off the list and try working on it.