If it was me it would have to be New Zealand. Plane tickets to there cost a fortune and it seems to be a lovely place to visit. The scenery is magnificent, a Kiwi friend of mine has been all over the world and says nowhere compares.
yeah i would say Australia and New Zealand, like others said you can have two stop overs each way, i would stop in Thailand then travel round South East Asia, and then Hong Kong so you can travel by train to China. Also Fiji if you can.
You'd need longer than 18 days though, if you have a free ticket make the most of it and go for at least 6mths.
August is midwinter in NZ so you may not see it at its best through the rain. Any place in southern Europe or the USA is likely to be uncomfortably hot. I'd go for the Rockies. Either in the USA, driving around Jackson Hole and Yellowstone, or in Canada - maybe flying to Vancouver, driving up to Lake Louise, Banff and Jasper, then down to Calgary for a look at the plains and dinosaur country. I've done both these and enjoyed them. You might even squeeze both into 18 days.
More specifically, I am a soldier in Baghdad and this is the 18 day leave we receive; so no expanding the timeline. Also, it would have to take flights from Kuwait and I would have to fly in and out of the same city.