I would plan on 5 or 6 per person so that they can be creative and festive with their leis. If you are really talented, have a ukulele on hand to serenade your guests as they enter. Maybe put a note on the basket urging them to wear them wherever they please. The backyard is just as fun to decorate as the rest of the house, so don't neglect this space if weather permits or if you plan to expand the party outside. Paper lanterns might be a nice touch, whether inside or in the back yard.
Tribal masks are fun, too.
Hang wind chimes and wooden parrots to give a complete background to your setting. Shells, sand and perhaps a mini-pool is order here as well. Candle votives are a must, but don't forget the citronella candles. They make for a great nametag or seating card. Line them up alphabetically on a table near the entrance.