Just get a cheap pre-pay phone. You can get em for around A$150 and it's much cheaper than international roaming.
Don't just go to the beaches, head inland if you've got the time - remember Australia is pretty much the same size as the contiguous US so places that look far apart, are far apart.
It's heading into summer down here now so expect some high temps, not too bad at the moment. As it's spring there can also be cold outbreaks so pack a pair of trousers and a light jumper (sweater) just in case.
If you're headed to the outback and are driving, avoid driving at night at all costs - it's far too dangerous with animals crossing the roads unless you're driving a road train and can brush all aside (including small cars).
Sydney CBD is relatively small but greater Sydney houses 4 million people so it's pretty busy traffic-wise. Study the maps, pull over if you need to and if all else fails ask someone.
Do the touristy things - they're pretty good. Climb the Harbour Bridge if you can afford it.
Most hotels have WiFi, there are internet cafés everywhere so connectivity isn't really a problem.
Just enjoy it, the weather's beautiful, the beaches are really starting to look inviting after a pretty cool winter and there's always something to do.
