Must do, take a 'City Sights' bus tour to get to grips with the city. And if you're lucky you have a fabulous busdriver, who goes the 'extra mile', literally!
In between you can jump off and back on again at any stop. In the evening, use your City Sights ticket to jump on the CityCat ferry (it's free, or rather included), back up the river, at night. Great view of Brisbane skyline at night.