Port Douglas makes for a nice resort, but if you can get a slightly more premium room at the Hilton in Cairns then you'll at least be able to relax, unwind and get comfortable in the bathtub.

That said, if you'd prefer to get active then a day-trip snorkelling on the Saturday would make for a grand affair and also enable you to enjoy the major sites on Friday and Sunday before you head back to the airport. My old folks went out to the reef on such a day-trip and they haven't stopped raving about it since.