If this party will be in a region of the country where it will be warm and you can have it outside, then you could definitely put up tiki torches. If you have to have it indoors, you can still decorate the place like you are on an island, with those mesh covered glass candle holders, and other tiki-related items (that can easily be found at a party store). And you or a bakery could decorate a flat sheet cake like an island in the middle of the ocean with an edible palm tree sticking up out of it. Serve a tropical meal, which can easily be found at a Hawaiian barbecue place (they’re all over California). Come up with some question and answer games that have answers that are based on the friends and families of the bride and groom and have it end up with the loser of the game being “voted off the island” and the winner getting some kind of island prize. You could play “pin the coconut on the palm tree”.Give out plastic leis to the guests and serve tropical drinks (alcoholic or non-alcoholic).