Take a CityCat ferry along the Brisbane River for cheap sightseeing

Swim at the free Streets Beach at South Bank Parklands (Australia's only inner-city artificial beach)

Day trip to North Stradbroke Island for snorkelling, surfing, and wild dolphins

Visit Lone Pine Koala Sanctuary -- the world's oldest and largest koala sanctuary ($49 AUD entry)

Climb the Kangaroo Point Cliffs or abseil down them ($45 AUD)