There is (or used to be) an official Order of Old Bastards!
In addition to what everyone else has said, much of which can be fairly regional...
Another Boxing Day tradition is the start of the Sydney to Hobart yacht race. Living near Hobart, this is huge - many people pour into Hobart and crowd around the waterfront area, as boats trickle in.
We do take our sports seriously, but mostly professional sports types as opposed to high school / college teams. There are, for example, several forms of football played here - Australian Rules, Rugby Union, Rugby Leage and soccer. There are national, state and local area competitions.
Holidays tend to be fairly laid back affairs, particularly if the weather is good. When possible, many of us love to spend time outdoors. Barbecues are wonderful.
I tend to find us Aussies are friendly but a little reserved compared to some of our US counterparts - when I was in the US it was not uncommon for total strangers to approach and talk to me, which isn't always the norm here. I've had the same experience in Aus when on long motorcycle tours but generally it doesn't seem common. I admit, I'm reserved with strangers, but I can also quite happily chat with strangers without any problem at all. I must have an approachable face, I'm always the one that people ask for directions... even in a strange US city (and I could help too!)
Some of us like to exaggerate a bit too, and tell tall tales, particularly to visitors. Hence the tales of hoop snakes, drop bears and fangaroos. But conversely, if you fly to Sydney and think you can bicycle to Uluru in a day, we will try to correct you - it's one thing to make a joke but we don't want anyone to come to grief because of it.
Distance here is very different - our continental land mass is nearly the same as the USA but we only have 6 states and 2 territories. So driving from state capital to state capital is a big, big trip. And if you want to visit Tasmania, it's fly or ferry.
Food is very different. Lots of Asian flavours and ingredients have become the norm over the past years. And our supermarket brands are very different to what's available in the USA. (I nearly had a conniption in a Walmart - wow, so much stuff, so different, what the heck am I buying??