I haven't been yet, but am planning on (like I tried to do about 4 years ago) going to Australia - the land down under.
There is MUCH to see there. It's a huge continent!!! You have all sorts of terrain - and weather - from one side to the other. You need to remember that their seasons are opposite from ours. The best time (in my opinion) to go would be around spring or fall. From what I researched - the rates for air-fare are at their lowest at these times.
Australia - I believe - now has a train that runs up the center... starting somewhere down near Adelaide, and going up to Alice Springs... and beyond.
http://www.seat61.com/Australia.htm
I dunno... sounds good to me... of course... I'm more of a 'I wanna ride the train' type of person.
Don't forget to get your passports... you'll definitely need those even if you visit our neighbors to the south (Mexico) or north (Canada).
I would say - do your research online... and when you narrow it down - try to locate a person who lives in the locale that you are interested in visiting - and have them send you brochures from their area.
Good Luck - and Have Fun!!
Regards,
Jim TX