object-oriented design – What are the user-friendly tools for prototyping conceptual data models?

I am in the early stages of planning a potentially large project, and I am looking to prototype the basic data structures involved before getting bogged down in coding all of the behavior / logic.

Ideally, I'm looking for a GUI application with a declarative interface that will allow me to define arbitrary data types, specify hierarchical relationships between them, store instances / records of these custom types and export these models / games data in tabular / serialized formats.

I have considered writing JSON or YAML in a text editor or using a spreadsheet to approximate a relational database, but neither of the two approaches really does a great job of capturing / visualization of relationships between objects. This makes sense given that data serialization is the point of JSON and YAML, and a downside of relational databases is that associative tables are much less readable than just having a nested / embedded list in the main object record.

I am aware that there are many conceptual / mind mapping tools out there, but from what I can tell, these tend to be less structured than what I generally look for. Is it time for me to choose an appropriate modeling language like UML / SysML, or is it overkill if I am not trying to model behavior? The only other closest thing I can think of is like a wiki thing type where you can define templates for different record types, and link the records / instances of those types, but obviously I don't want to mess with all the headaches of HTML if I don't need them.

Obviously, my code will eventually be the real representation of this data model, but are there quick and practical visual tools to prototype this kind of thing that I neglect?