Last time I was in one of those party stores, there were plenty of Hawaiian/tropical/luau decorations and party items.
For food, you can serve roast pork over plain white rice. You can cook it in a crock pot, no need to cook the whole pig in a pit! Or you could grill something like teriyaki chicken or beef, in pieces or on skewers. Fruit salad, pineapple, fruit punch or iced tea. Kona coffee, coffee ice cream. Coconut sorbet, cookies. Macaroni salad. For some reason, this is popular at the Hawaiian eateries around here. Hawaiian sweet bread or rolls are good and easy if you can find them in your area.
You should be able to order fake flower leis online. Not the scratchy ones made from plastic. There are some inexpensive pretty ones made from silk flowers that would be fun to pass out to guests.