Invitations
You can buy cardstock and print out a nice Hawaiian flower.
Another idea is taking a picture of the celebrant with a Hawaiian attire or hula skirt.
If you know how to edit photos in the web, you change the background to match your theme.
If not you can take the picture by letting your birthday celebrant sit on a sand box
and a fake palm behind. Inside the invitations write the details of your party.

Decorations
Don’t forget to make leis for each of your guest. That will be a nice effect for your party. Use some nice Hawaiian colors for your table cloth and napkins like green, yellow, blue and orange. You can find some inflatable palm trees and tiki pole and put them around the pool area. If you have a deck you can make it as a tiki hut or a patio with umbrella will do. Of course, you need some flowers. You can buy artificial ones at the craft store. Look for one close to Hawaiian flowers, orchids are good. You decorate your tables with a hula doll or a transparent vase and put colored sand and colorful shells. You can find those at the pet store.

Food and Beverages
The food should be all Luau-inspired. You can find great recipes on-line for Aloha chicken wings, Polynesian pork, Pineapple and Ham rice, Triple pineapple glazed chicken, Hawaiian style spareribs, Warm chicken salad with pineapple and hotdogs for the kids. Most of the foods have to be marinated the night before and then simply grilled right before eating.

You can make a birthday party. You can bake 2 13x9-inch cakes and frost them as one cake. You can draw a brown (chocolate frosting) volcano (erupting) in the blue water (colored white frosting), you could use brown sugar to simulate sand and draw a palm tree. Also use green-colored coconut for the palm leaves and 3 brown M 'n M's for the tree's coconuts. You could also write Happy Birthday in Hawaiian on the cake.

Games
Do the limbo… it so much fun for everyone.
Pass the coconut is a good game too. It is the same idea with hot potato game.
Prepare a hula piñata so the kids could use their pails to put their goodies in from the piñata.