if you hire a car go up the the karanda markets, or take the skyrail up there, huge shopping centre in the middle of cairns also to keep the misses happy
the mariner and resonably new lagoon (1mil refurb swimming area) is pretty good also.
use to be a place along the water called Charlies, and you could get $10 all you can eat seafood, but its been years since i was there and i dont even know if its still operating lol.
also if you hire a car, head up the mountain and see some of the daintree and national park areas, alot of them have awsome waterfall swimming areas.
Kind of depends what your into?