I have a trello board with a card for each project or blog post.
There’s 4 lists:
Triage: New ideas go here as I think of them.
Graveyard: Archive of bad ideas, with a description of why they are bad.
Needs: Potentially good ideas that need fleshing out.
Ready: Ideas where I’ve thought through and documented market fit, timeline, tech stack, marketing, or whatever the project needs. And crucially: the first thing to do when I start the project, which makes it easier to get going.
When I have free time I look down my Ready list and pick something interesting, and I’ve done the prep already to get stuck in straight away.