Learn English – Future meaning of “going to”. What does it exactly mean

She is going to spend her vacation in Hawaii.

There are two possible interpretations of this sentence according to grammar books.

1. intention
   The speaker expresses the woman's intention to spend her vacation (in British English ‘on holiday’) in a specific destination. The decision was taken at some point in the past, we do not know if the hotel or the flight are booked but when there is no time expression such as; next month, in December, next summer, we usually infer the immediate or near future.

Put simply, she has made the decision — in the past — to travel and spend her vacation in Hawaii

2. Prediction
   The person is about to spend her vacation on the Hawaiian islands. We base our prediction on something that can be seen, and therefore deduced. We might see the person in an airport with her boarding card, maybe she and her partner are wearing typical Hawaiian-style shirts. Or maybe she is actually boarding a Hawaiian Airline airbus which is scheduled to take off.

   

Usually a sentence similar to the OP's refers to the first meaning, but without context we can never be 100% certain.

References

Advanced Language Practice by Michael Vince

Unit 2 Tense Consolidation: future time

3. Be going to describes intentions or plans. At the moment of speaking the plans have already been made.

I'm going to wait here until Carol gets back

Going to is also used to describe an event whose cause is present or evident.

Look at that tree! It's going to fall.

Decisions expressed with going to refer to a more distant point in the future.

A Practical English Grammar Fourth Edition, by A.J.Thomson, A.V.Martinet

206 The be going to form used for prediction

A The be going to form can express the speaker's feeling of certainty. The time is usually not mentioned, but the action is expected to happen in the near or immediate future [...]
B 1
be going to implies that there are signs that something will happen