Queenstown bike park - no brainer. Get a DH rig and enjoy!
I second this. There are minimum 5 different bike shops in the city center that hire bikes. Whether you are after an Enduro bike or DH bike, there should be something available.
Seeing as you only have one day, id spend it at the Skyline gondola. This is the main bike park in Queenstown and is 2min walk from the city center. A full day here will be plenty. On your last run, if you are feeling good and have time left with the bike, catch the gondy to the top and then continue riding up Ben Lomond as there are some good tracks up there. The gondola will cut the climb in half.
Other tracks around the area are also great but you will waste hire time travelling to get to them. Skyline bike park is best bang for buck.
Just make sure you don't wear your self out too much so you have no energy for the nighttime activities! Your new wife might be a bit annoyed! If you need to make it up to her, go for dinner at the Nest Kitchen + Bar which is located at the Kamana Lakehouse hotel. You wont regret it.