How about tropical fruit platters,big bowl of fruity punch,and lots of finger foods.BBQ,
Maybe you could use large candles for decoration,how about some cheap fishing net and buy some cheap plastic fishes,crabs,sharks etc....from the cheap shops.
Bannanna lounges,you could set up a limbo sticks for some fun,play some groovy 60`s music. I?m sure your Dad will have a lovelt time.