Learn English – a word or phrase that means “both achievable and challenging?”

Fun on its own works fine for me—I would also categorise music and videos under it, but I would not expect to find music or videos on a website where a menu item is labelled ‘fun’. I would expect to find more or less interactive, amusing things, like games, jokes, cartoon strips, etc.

To add to the interactive dimension and more or less exclude music and video, perhaps fun and games could be used. That is a more or less set phrase that does not merely refer to anything that is fun + anything that is games, but rather (dictionary definition) to “amusing and enjoyable activities”.

Of course, it’s hard to tell if this is ideal for you without seeing what the rest of your site looks like; but it works well in general, I’d say.

No need to invent new words right yet, we have terms for many of these things!

One of the most commonly recognized scale-independent forms is the Fractal. It mains the same form whether 'big' or 'small'.

In fact, the concepts 'big' or 'small' don't even have a real connection to fractals, which gets at the essence of what you're thinking.

Generally, you are referring to scale-independent or scale-invariant phenomena.

You could use the word scaleless, but I find its awkward to many people.

Source: I have published an article about scale-invariance.