Hi Kim
Oh, how wonderful and utterly exciting - you will have a lovely time I'm sure!
First up, here are a couple of websites that you may find useful. I use these quite often to plan my trips. (Well admittedly, all bar the first - I kinda have an idea what Australia has on offer! ;-):
http://www.australia.com/index.aspx
http://www.virginblue.com.au/
http://www.wotif.com.au/
http://www.vroomvroomvroom.com.au/
http://www.qantas.com.au/
The biggest thing to be aware of is that January/February is the time of year where we often have sweltering heat. I'm not kidding when I mean hot - often over 30 degrees celsius and can be high humidity. Be aware January is also our summer school holidays, so you will find that you are paying peak prices (generally anyway) and may have to compete with families with children. (I just wanted to make sure that you wouldn't mind this since it is such an important anniversary.)
In terms of all round nice weather for Australia, I'd recommend March as my absolutely favourite month of the entire year. I find that it is pleasant pretty much the whole way around Australia. Humidity drops down up north, the south finally gets some reprieve from heat and it generally has nice, mild temps (still hot in comparison to you though!) and the kids are definately back at school . Otherwise, September/October tends to be pretty good 'round the country as well (there is a school holiday in the middle though).
If you came (say....) end of Feb/early March and headed up North to Queensland first, it would absolutely be swimsuit weather still and perfect for lying by the beach. I can recommend going to The Great Barrier Reef. If you go there, you MUST try and get to Whitehaven Beach - one of the most perfect beaches in the world! There are also gorgeous Islands to stay. Try Hayman Island for a luxurious celebration. I live in Brisbane as do some of the others - you must get here and say Hi! (Just thought I'd throw that in )
Sydney has the famous icons of the Harbour and the Sydney Opera House and the famous Manly and Bondi beaches. If I was going there for a few days I must confess I'd probably be inclined to try and stay on the harbour if possible - Darling Harbour is the central point around there (from what I recall).
For shopping and culture, I'd recommend Melbourne. A great city and reminds my English hubby of more of a European feel than the rest of the place.
Tasmania is stunning if you like nature - hiking - eco-tourism - convict history - that sort of stuff.
Northern Territory has Kakadu national park and Uluru. It is a pretty vast place, sparsely populated and a totally different perspective of Australia.
I'd say when planning, the biggest thing you need to remember is that Australia is a HUGE country. For instance, if I travel from Brisbane to Canberra by car, it will take me *over* 18 hours (I leave at 4 in the morning and will drive for a full day till 5pm and then do the remainder few hours the next day - it is not for the light hearted!). Canberra to Melbourne is around 9 hours by car. (That's why I often fly if I can!)
When you have a better idea of the States and things you'd like to find out more about, please let us know and I'm sure we would be absolutely delighted to help and make recommendations.
Happy planning!
x