Mindmaps can be used to structure ideas and visualise relationships.

Mindmeister - is a visually very appealing and intuitive tool (requires registration)

Other tools that can be used to create a mind map:

Padlet - offers with many ways to organise information(guide).

Flinga - kept simple and therefore very intuitive(guide)

Miro lite can be used as a whiteboard, pinboard or mindap, also works without registration and is GDPR-compliant.