Hey Matt.
I'm on the other coast but if I wasn't booked up this weekend we could have hooked up. Funny last time I was in Edinburgh, this time last year, I was showing an Aussie girl around.
The castle is always good (try and be there for the 1 o'clock gun if you can), have wander along the Royal Mile, down into the park on Princes Street. There should be loads of stuff to see and do, loads of street performers, shows etc since the festival is on. You might be able to pick-up tickets for the Military Tattoo up at the castle if it is not sold out, it's the last night on Saturday. There's no Aussie regiments there this year but there are some Kiwi's there so you could go and heckle them
I hope you've got your accomodation sorted, it'll be chocka. It's a holiday weekend up here as well.
Oh and don't forget a visit along Rose Street, jam packed with pubs and bars.