Dandelions might work, and would be worth experimenting with. Count on several dozen specimens per lei. When your guests arrive, make them feel like they just stepped off the plane on The Big Island. Someone should play the congas for authenticity. Prior to the Luau, look up your guests' names in Hawaiian and write them on blank name tags.
Luaus are casual, comfortable affairs.
Live entertainment is always best, so tap into the talents of your guests. With the right percussion, your guests won't be able to resist joining in on a graceful, dignified hula dance. Pi atas are fun for all ages, if filled with age-appropriate trinkets. Rent a hot tub for the event. Have a special category of awards for non-alcoholic entries.