I'm going to a party [on Friday night].
We're heading to America for a holiday [this summer].
Remember, though, that of course the present continuous in your example could mean that you are actually currently in the process of making/buying/pouring a coffee. The timeframe or the context would make it clear which one you mean.
However, if it's the reply to a question, one or the other might be appropriate:
What are you doing?
I'm getting some coffee.
What are you going to do this afternoon?
I'm going to get some coffee.
What will you do later?
I'll get some coffee.