You can serve Mai Tais. They seems to be at every luau. You can also serve just about any tropical drink. Pina Coladas, Painkillers, Sea Breeze, Bay Breeze, just about anything that has a fruit juice in it will be a hit!
Any fruity drink would work. I would suggest a sea breeze for the ladies. It is an ultra sweet drink with a coconut flavor. Cororna would do for the guys.