Answers and Explanations:

• The surprise party isn't going as planned!

  "The surprise party" is a singular subject (it), so we use isn't with the "-ing" verb (going).

• I haven't got the balloons yet...

  We use haven't got to show that we do not possess or hold something.

• ...and the guests don't know where to park their cars.

  "The guests" is a plural subject (they). We use don't with the base verb "know" for plural subjects in the present simple tense.