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.