I love Queenstown in winter and spend as much time there as possible. But this is only because I really enjoy skiing and wine. There's some great ski fields and many wineries (particularly good Pinot Noir) so if it's good weather I'll ski and if it's not so good we'll drive around the wineries and replenish our stocks of wine.

If you aren't into either of these then maybe you could remove a day. You do want a good day to goto Milford Sound and the weather can be a little variable so a couple of days to choose from would improve your odds (Arrowtown is a good option for a dull weather day).

There are plenty of adventure sports available year round - Bungy Jumping (if you aren't that adventurous there's a bungy-swing too), Jet boating is good, quad biking (or snowmobiles in winter).

My favourite restaurant there is the Bunker and I really like the Sofitel if you're looking for accomodation (not a great view but stunning rooms).

PS Try and fly on NZ rather than QF - QF seems to divert flights to ZQN at the hint of a cloud in winter.