Symphony.

Ok, I have decided that I want to be able to know how to do this. Apparently you don’t need to know much php to do it, so I feel like I am in with a shout.

Any advice on something simple to create as an exercise?

I started with field types. They’re very simple.

What like creating a field type?

For example creating a radio button field for entries?

Sure. Have a look here for some other field type extensions.