In my opinion, the best lighthouses in Hawaii are on Oahu (particularly Diamond Head and Makapuu) followed by Kauai (particularly Kilawea). I agree with some others that seeing too many islands in one trip isn't the most relaxing trip. Therefore, I would recommend spending 2 or 3 days on Oahu and the remainder on Kauai. I also agree with one of the above posts about staying in the Poipu Beach area of Kauai - better weather, good beaches and lots of restaurants and other stuff to do.

Have fun and aloha!

Larry