I went to Queenstown last year early September and found the snowboarding was pretty good so it will be great in August.
We were able to do all the extreme sports. We did all of what you want to do and more. They do say however that rafting in best in summer as there is heaps more water, but we found it to be okay.
A great place in Queenstown for nightlife is The World Bar (For a Teapot) right in the centre street. It's pretty ecclectic and the roof opens up some nights, but maybe not when your there as it's winter. Also try Minus 5 bar, it's a pricy entry fee @ $30 and you can only stay 1/2 hr at a time but you get 1 cocktail inc. and it's worth a look.
I would recommed a skiydive over Lake Wakatipu in Queenstown. I've done a few dives around the world and it was the most beautiful place to do it. Very Very Cold up at 15000ft though. Queentowns is VERY expenisve, I'm from Brisbane so used to high prices for drinks - food - rent etc but this place is exxy. My boyfirend and I were there for a week and spent about $3000 between us. Although we did skydive, bungee jump, jetboat, luge, rafting, helicopter ride, lots of drinks at night. But that didn't include any accomodationa as we had a campvan. Prepare to spend up big.
I didn't find Christchurch that great at all. It's just like being in any other capital city not that different from Australia. It also rained when we were in Christchurch so that made it a bit more miserable.
How are you planning to get from C/Church to Qnstwn? We had a campervan, and the drive was really beautiful, with plenty of places to stop and do day hikes along the way, if your after that sort of stuff.