If you have a swimming pool at the venue, then you can make some tropical-inspired floats on the water; you can even let real flowers float on the pool water for complete Hawaiian decorations. Such luau decorations are also great for small ponds.
Lastly, be sure that coconuts and pineapples have a place in your Hawaiian party. You can serve chilled drinks in coconut husks and serve pineapples in a fruit plate or as an ingredient in the kebab. You can also, of course, use pineapples and coconuts as decorations.