I have a trello board with a card for each idea. I have 4 lists:

- Braindump: New ideas go here as I think of them.

- Graveyard: Archive of bad ideas.

- Research: 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: 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.