Randomly think of a thing. Let it bump around your head a bit. If the bumping gets too loud, start writing the words with the nearest writing device. See how far you get. The more words usually mean a higher degree of personal interest. Stop when it suits you.
This is exactly what I should do more. I often have random ideas but never get them into Ulysses because they are just thoughts. Maybe I should keep way more drafts and see where they lead me.
(Instead of Agenda Ulysses can be used for journaling as well. Maybe I should keep everything in one app.)