I agree that Wanaka is a good place to go
Johnm,
Let's face it, ANYWHERE in NZ is a good place to go flying.
destinationsky,
Besides the options already suggested, you might want to consider going to Omarama (north of Wanaka) and doing some mountain soaring in one of their two-seaters. If you book in advance, you might be able to fly one of their DuoDiscus - a wonderful aircraft. On a good day, you might be as far as 100 km from home before turning around. I made to it 16,000' over Mt. Cook.
Goto
http://www.glideomarama.com/adventur...glider-flights
and look at the Long Flights option