The weekend before my birthday I spent in Tairua on the Coromandel Peninsula with a couple of friends.

About 40 minutes from Tairua is Hot Water Beach. In an area of about 20m on this beach there are hot currents under the sand which can be used to have a warm bath during low tide. You should bring a shovel though because in some places the sand is just too hot to move it just with your hands.
However, like proper tourist we were badly prepared and did not bring any tools to help us dig a pool. Instead we just waited until somebody else left their puddle and then jumped in. The water must have been close to 40 degrees Celcius so it was just perfect. If it got too hot people ran into the waves of the Pacific Ocean and as soon as they started to feel cold they just returned to the hot pools. I could have spent the whole day like this without getting cold but after a while we started to get bored and returned to the bach in Tairua...