Arriving at Lake Reynisvatn (there is parking, or bus 18 stops there), walk along the lake to the left. You will see a small forest of tall pine trees. There is a trodden path towards the forest. Follow it into the forest. There is a clearing. Climb up the hill and enjoy the crunch of pine cones under your feet. The cache is close to the top. I recommend you continue up, and then turn towards the lake before you go back down - there is a great view.
Please hide the cache well, this is a popular spot and we don´t want muggles to find it