Hi coll, If you will do a search on the message boards for "luau party", it will bring up a lot of different threads with tons of really good ideas.
I was going to post one for you but then I realized that they are spread out and it would just be easier for you to do the search that way you can look at each thread.
Have a great time with your party! It sounds like such a fun theme.
One more thing...... I was in the Dollar Tree at the start of summer and they had a pretty big selection of luau decorations over by the party supplies.
Not sure if they still have anything but it might be worth a try.
Many party stores should be starting to have sales on this theme now that everyone is focusing on school and Autumn already.