One of the tools I use for planning and mindmapping is a diagramming tool.
There are many great options out there such Omnigraffle and Lucid Chart.
However, one option I recently discovered is draw.io.
Draw.io is similar to Lucid Chart except you can use it for free. This is especially helpful if all you need is the occasional tool for creating a quick diagram or updating an existing one.
Let me know what you think!