The HMAS Brisbane was scuttled off Mooloolaba in 2005, and is now a well regarded wreck dive and marine reserve. Several dive operators run trips there, and for much of the year they do both morning and afternoon dives.
Starting from Brisbane, using public transport, is it possible to get up to Mooloolaba for about 11.30am (in time for an afternoon dive), and return to Brisbane after about 5pm (when the dive boat returns), thus doing it in a day? Or is the only option to stay over on the Sunshine Coast either before or after the dive?