Ok, I'm a cruising junkie so over some time I've picked up the following from other cruisers and use these tips now when I cruise. You can take or leave whichever ones you feel will be of benefit to your trip.
I tend to take my own towel with me anyway, as PP's have said, the ship will provide you shore excursion and pool towels but they are no bigger than the regular bath towels in your cabin and I love my huge beach towel so I just pack it anyway. I pack it over the top of everything else in my suitcase and tuck it in the sides so it is a barrier to anything moving around while in transit.
When it comes to clothes, check if your cruise has any formal nights or if there is a themed party on board. Even if you don't think you'll want to attend these before you go, pack for them just in case. It's next to impossible to find something to wear for the occasion on board if you change your mind - and they are actually a lot of fun.
I'd pack your toiletries in a hanging organiser that can be hooked on the back of the bathroom door - frees up the small vanities usually found on cruise ships. The cruise I am doing next October is on the Celebrity Solstice and by the looks of the pictures, the bathrooms there have the most storage I've ever seen on a ship so I'm looking forward to using shelves for the first time and actually having some room to put everything.
Check to power outlets on board (it should say in the specs of the ship on your cruise website) to make sure it's Australian plugs and that you don't need an adaptor to American or European outlets. Take a power board with you so you can plug it in the one outlet that will be in your room and then have multiple power sources for cameras, laptops, phones etc. Lack of power points in the past has meant that DH and I are fighting over the power point in the bathroom so he can shave and I can charge the camera!
I pack a travel clothes line with pegs to string across the balcony when I have that category cabin. It's the best way to dry my towel at the end of a day of swimming so that it's ready for the next day.
One idea I heard on the last cruise and am looking forward to trying next time is to pack the centre tube from a glad wrap or foil. When you get commemorative cards, photos etc you can roll them and slide them into the cardboard tube and then they won't be creased or crushed on the way home. I got my photos last cruise in their cardboard folder and during transit from the Bahamas, to New York, to Vegas, to Hawaii and then back home, even in the folder the photos got dented and bent. If I had known about the tube, I probably wouldn't have had that issue. The photos would be curling up when you take them out but put a phone book on them or frame them and they will go back to being flat and in original pristine condition.
That's all I can remember right now. It may seem like a lot but it makes the trip all the more easy and I've packed all this stuff before and still gone away with just a carry on bag. It's not stuff that takes up a lot of room, plus the rest of your clothing will be light Summer wear and so will be lightweight and small to pack.
If I think of anything else or you have any other q's I'd be happy to help. I wrote a couple of posts about cruise tips on my blog too if you want to check it out.
I hope you have a great trip!