The boulders on Moeraki Beach are most beautiful indeed. I was fortunate to visit during sunset and sunrise at low-tide. The boulders are fascinating - some perfectly spherical, some cracked, some completely destroyed but all worth seeing. Visiting outside of the usual daylight hours, it wasn't actually that busy which made it all the more enjoyable. Well worth turning off SH1 if you're driving past.
Allow some time to visit the seals and lighthouse at Katiki Point. There's a short walk you can do out along the peninsula - if there aren't any seals blocking the path. Alternatively, drive on down to Shag Point and maybe you'll even spot some penguins.