Well there is plenty to do in Brisbane, and its surrounding areas.
In terms of major things to do in Brisbane, take a river cruise from the CBD to Lone Pine Koala Sanctuary. Southbank has the Cultural Centre (a number of different places) which always have some free exhibitions, markets, live music, performances etc going on.
From Brisbane you can explore Stradbroke or Moreton Islands with regular ferries heading over as a day trip (but good to spend at least 1 night there I think). North Stradbroke is the most developed and you can drive around the island easily...the others are if your up for a bit more nature or camping experience!
You can climb the Story Bridge which is a cool experience if your OK with heights
Theres also Mount Tamborine (south on the Gold Coast, 1 hr drive from Brisbane) there are amazing views, rolling mountains, cool bushwalks and waterfalls, and some nice boutique breweries and cheese makers for lunch. You can also easily get to the main beaches on the Gold Coast from Brisbane (again an hour drive).
Hope that helps
We run some unique day tours in Brisbane (called Brisbane Urban Adventures - just Google us) and are adding some more tours shortly focusing on guided bike tours, and also lunch/dinner cruise options with the Kookaburra Queen paddle boats - happy to answer any other questions you may have.