amber,
I agree with the comments from the others. With such a very short holiday, limiting your trip to one country would be prudent. If you only have 14 days total, that will leave 12 days for actual touring once the flight days are deducted. On top of that you'll be dealing with jet lag for the first few days, so may not be up to full "touring speed". One other point to keep in mind is that your trip from Italy to Spain would use one full day, so that reduces your touring time further.
As the others have mentioned, the Amalfi Coast area is the "outlier" and will take more time to get about there as transportation is not as well developed as in the north of Italy. The "easiest way to travel" is usually by train, especially the high speed versions which run at up to 300 km/h. "How long does it take" will depend on which route you're travelling, and whether there are any changes. I gather this is your first trip to Italy, and also should mention that there are some potentially expensive "caveats" to be aware of when using trains and other public transit there. If you need more information on that, post another note.
If you do decide to limit your trip this time to Italy, you should be able to add one more destination to the list. Some that you could consider would be Florence or Rome. Florence would be easy, as it's on the way from Venice to the Cinque Terre. Rome is probably your best bet to find good flights back to Australia.
There are many possibilities. As others have mentioned, it would help to know when this trip is taking place. I'll leave the question on "night life" to the others, as I'm considerably older than 22 and not exactly "in tune" with night life these days.