Float candles in bowls of colored water or in a pool. If you don't have a pool, purchase a kiddie pool and camouflage it in chicken wire and cover the wire with leaves and plants. If you can't rent them, then get a cassette of bird songs and stick it up in a tree to play over and over. Use big shells or wooden bowls as serving dishes. And dont't forget the little bamboo umbrellas for your beverages.
Nothing brings a luau to that special high than a than the dances of Polynesia.
However, if your area doesn't offer this sort of thing, find some smooth easy listening lyrics to set the scene. Also some sounds of the ocean would be a great mood setter. You can also find some wonderful Hawaiian music on-line. Just to show them what it would lQQk like, if they were in the islands. You can most likely rent them at your local video store.