You didn't say where your point of origin is so... I'll just call it DFW Airport for argument's sake.
With 8 nights away, you've got about 1 day travel there and 1 day travel back. So, that gives us 6 nights "on station" over your primary area of interest.
You don't want to put yourself in some sort of situation where you might miss a flight connection and get stuck so, I'd narrow it down to destinations that you can get to non-stop from Dallas/Houston. Unless you want to intentionally have a day or two stopover on the way and then non-stop back. For example, spend two nights in Hawaii or Los Angeles on the way to Japan and then non-stop on the way home.
You've already stated that you want a full two weeks for a trip "Down Under". Fair enough.
I assume you'll be ruling out places in the Middle East for security concerns and skipping things like the Abu Dhabi 48 hour challenge.
So, that leaves Far East Asia, South America and Central/Northern Europe.
Asia- Japan - first choice, Tokyo, Kyoto, Osaka, Okinawa, etc.
- South Korea - seriously, Seoul is awesome
- People's Republic of China - a bit of an environmental/health concern but, Beijing, Shanghai and Hong Kong are all must see at some point
- Republic of China (Taiwan) - Taipei is interesting and more interesting if you get further afield.
- Singapore - fun but... probably not what you're after - imagine Houston
- Thailand - didn't do much for me but, I'm not a young single G.I. either.
South America - I'd love to go to all of these
Central/Northern Europe- Amsterdam/Holland - Plenty to see/do and easy to go to other places
- Paris/France - same
- Germany - unlimited amounts of stuff to see/do
- Scotland - especially because I've been watching the BBC Programme Shetland, I'm keen to go to Scotland now.
Those are my initial thoughts. Pick one destination and go there for 5-6 days. Or... pick two destinations that are close together and do 3 days in each - i.e. Amsterdam/Paris or Rio de Janiero/Buenos Aires
Good luck and let us know what you decide to do.