Having been to Christchurch and on the jet boat ride during our overland excursion to Queenstown, I second Ngaire's idea. (I may be a little biased because our day in Christchurch was accompanied by somewhat dreary weather.) But we sure had fun on the jet boat ride the day we did it; I was all ready to do it again.

I did a quick Google search and saw several jet boat excursions around Christchurch.

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Eugene