When I was in school we called them "webs", now they are mind maps. No matter what you call them, they can be a fun way to get ideas out of your head and into something visual that you can share and use as a planning reference. Mindmeister is a good app that is available on a variety of devices. https://www.mindmeister.com/

They do offer a paid tier, but the free product works well for most classroom activities. Students will need an account, but they can sign in with their Google Apps accounts and if you are using Chrome browser and signed in there is an extension that can be allowed access to your basic authentication info and will sign you in automatically.