Go for a swim at the ever popular Undercliff on the Tukituki River.
A short distance down the Te Mata peak from the car park is a cave that is rumoured to extend for kilometers. People have estimated it ends somewhere underneath Hereworth School. It's slightly difficult to find and would be very dangerous during rain. This should only be for experienced cavers. To find it: Go to the redwoods car park just after the stone gates. Go down the hill to your right (facing away from road) through the trees folling the bottom of the valley. It's about 300m down the hill and you will need to jump a fence. Keep following the gully and you will know you are there when you see a rusted green Ford Escort. Make sure you let someone know where you are going, just in case.