How about…

Create a new list, called Scratch Pad. Every new item becomes an entry there.

Or create a new task, called Scratch Pad. Every new idea becomes a note in that task.

Or a Google Docs document?

I was trying to get away from structured entities like tasks. I sometimes find it helpful to have a free-format 'doodle pad' available. I'm not a fan of Google docs, just because I often work from a laptop with a 3g card and performance is poor to awful. Better to associate such a facility with RTM to minimise the number of windows necessary, and to use Gears.