Pokai Bay was decent when I went, it was good for stand up paddle boards when I went. I haven't gone yet, but I hear good things about the snorkeling at Electric beach and shells at Tracks (sometimes considered a questionable area, but I know people who really like it there, too. I've never had trouble anywhere on the island... As with any other place, don't plan on leaving valuables in your car/on the beach)
All of those would be closer than a drive to the north shore.