notion and idea

Nothing, they are synonymous.

Only difference I'd note is this:

You don't often find notions in written form. (typically if it is written down people no longer call it a notion)

But maybe that's just me...

The word 'idea' is used more often, not only for written work, but also in speech. This may be because the word 'notion' is considered slightly antiquated, thus it is often used in a whimsical manner. Those who commonly use it in everyday speech might give the impression that they are a little old fashioned.

Also, I would call "idea" a shade more serious than "notion".

Glad we agree!