I'm glad you're getting some time to see the country! That area is beautiful! I like Wakatipu - the drive from Queenstown to Glenorchy is fantastic. People like Wanaka, too, but it's more developed than Tekapo.
Queenstown being very developed (and expensive) means you can get some nice lake cruises and even a flight over to Milford Sound to do a cruise in the Sound. Yes, the flights are costly, but if you hit a fine day, it's an unbeatable scenic experience.