We just did this and it can be done easily in a day. Funny, I found the northern half to be faster. Check a good local map for a shortcut because we went further north than I believe we needed to. (Well, we were in Waikoloa....if you want to see the North shore, then don't take the shortcut )

I'd suggest a stop in Punalu'u at the Black Sand Beach. It wasn't far off the highway, had facilities, and was just beautiful. The view, the water, the beach....and my kids absolutely went crazy over swimming with all of the Hawksbill turtles that come up to the shoreline there. Honestly, it was as exciting for them as swimming with the dolphins, and much less expensive.