I’m looking for something that allows me to drop in ideas and move them around.
Well that’s exactly what mind-mapping tools are designed for. But since that is not what you want, I suppose I am confused by your question. So here are some things we use when planning and tracking software projects:
UML modelling software is similar to mind-mapping software, but way less flexible and requires a bit of understanding of UML before it will be useful to you. StarUML, WhiteStarUML are a few I have used. There are many others. Mostly used for planning software designs at a detail level.
There are also text-based tools, ranging from plain text editors to powerful wiki software. This could range from basic Notepad all the way up to tools such as Confluence. Typically used for documentation, but sometimes used to plan projects; at least in the early stages.
Then there are ticket-based systems like Jira, and card-based systems like Trello that allow you to create “lists” of things (tasks, features, bugs) and track their state / progress. These are usually used during development, but can also be used for planning as well.
If none of this is what you are looking for, could you try to clarify?