There are leis available at the airport if you want to buy yourself one once you arrive. There is a shop in the baggage claim area and also a whole row of stands right outside the Hawaiian Air terminal.
We were greeted w/an orchid lei when we boarded the Summit in Hawaii. It was such a nice touch and totally unexpected. Made us feel we were truly in Hawaii. I guess NCL thinks it would be too expensive to do on a weekly basis.