If you get out to Lake Tekapo, be sure to check out the Mt John observatory. I went there during my honeymoon in the late 90's and the chap who operated the site was eager to show us around. He would've even opened up a dome, but it was blowing 100km/h winds outside so he wasn't able to.
Otherwise, have a good look around the South Island and enjoy yourself. Highlights include Milford/Doubtful Sound, Mt Cook, Fox Glacier, Blenheim wineries (I've been twice in the last year!) and the wild West Coast. If you're into wildlife, the East Coast has tonnes of seals/whales/dolphins/birds.
Sorry, I'm rambling...
