I live in North Queensland and am happy to try and answer your question. From Brisbane, do the day coach tour to the Gold Coast (approx 1 hour south of the city). Alternativly why not do the day tour to Australia Zoo (Steve Irwins place) which is approx 1 hour north of Brisbane. They are very well set up and professional. You wll see crocs, snakes, kangaroos, wombats, tigers and tropical birds.
Am not sure where you will be calling into in the Great Barrier Reef area, most likely Whitsunday Islands or Yorkeys Beach near Cairns. From either of these places you can do a fabulous day tour to the reef where you will see all the marine life through glass bottom boats, submarines and the like. You wont need to snorkel if you dont wish to, you wont even get you feet wet! Cairns has an aerial cableway that takes you above the tropical forest up to Kuranda. From there you can catch a real old train back down to Cairns. Also from Yorkeys you can do a one hour trip north to Port Douglas which has to be the prettiest place on earth...right where the tropical rain forest meets the long white beaches. Whatever you choose, enjoy!