It's from cambridge dictionary.

No it's fine as is. except here is a conjunction meaning "with this exception"

She did nothing except watch TV all day.

Excellent observation. The verb form to use after a preposition is the -ing form.
But as pointed out above, except doesn't function as a preposition.

except, but, except to, and (rarely) but to are used the same way.
So are more than (to), less than (to), and similar expressions.

Compare:

She did everything except draw

Compare:

She did everything except draw pictures (to make him understand).
She did everything except to draw pictures.
She did everything but draw pictures.
She did everything but to draw pictures. [Hardly ever used.]
She did no more than (to) draw pictures.
She did nothing less than (to) draw pictures.
All she did was (to) draw pictures.

Ar

Are the sentences above different from each other? They seem all the same to me..

Yes. Look carefully. They have different words in them.

Do the sentences above mean differently from each other? They mean all the same to me..

How would I know when to use an ing-form after except?

Is The government has few options except keep interest rates high the same as The government has few options except keeping interest rates high?

How would I know when to use an ing-form after except? Is The government has few options except keep interest rates high the same as The government has few options except keeping interest rates high?

The key element is a form of do: She did nothing except loll about in the garden all day.
I should have emphasiz