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Take a surf lesson in Sumner!
Go on one of the mountain bike trails in the Port Hills (or wait for the new mega bike park to open very soon.)
Take up climbing - find a buddy and get out and about.
Some nice graffiti to look at - download the Streetwise app to help you find them.
If you have your own transport (or hire a car if you don't) - get out and see Castle Hill boulders / get a wetsuit and go through Cave Stream. Do a day trip up Avalanche Peak (perfect time of year for it). Check out Akaroa and the bays down on the banks peninsula - take a picnic and go exploring!