why not PM Keith - he's a local.
As an occasional holiday visitor up that way, I'll second the O'Reillys/Green Mountains/Lamington National Park/Binna Burra/Natural Arch areas in the Gold Coast hinterland plateau, if you like rainforest, mountain vistas, waterfalls, birds (often very tame) and rural scenes. A few hours to get there, book ahead for Binna Burra or O'Reillys accommodation - check their web sites.
A few hours drive north of Brisbane is Fraser Island National Park - an amazing place, world's largest sand island with rainforest, white sand, freshwater lakes and beaches. There are wild dogs (dingoes) which you need to be cautious about. Take a tour or hire a 4WD as the tracks on the island are deep sand in places. Google Fraser Island for photos.
September is spring down under and it is getting warm but not too hot to explore inland. Another interesting scenic place is Carnarvon Gorge National Park
http://www.derm.qld.gov.au/parks/carnarvon-gorge/index.html, allocate at least one full day to explore plus at least a 5 hour drive from Brisbane. There is Aboriginal art on some of the rock walls in the gorge. Last 20km of dirt road is no problem when dry but can be 4WD only after rain, it gets very slippery and muddy. Accommodation is hard to find.
Northern NSW is a few hours drive south of Brisbane. Australia's most easterly point is the beautiful Byron Bay lighthouse, Byron Bay is a pretty resort town but quite commercial. Just inland from there is Mount Warning - an old volcanic plug surrounded by a ring of mountains which is what remains of the caldera. This is a very scenic area. Mount warning itself can be climbed, but the drive around the western rim of the caldera has some great views to the coast. You can get to Mt Warning and Byron from the Gold Coast hinterland by taking the very scenic Numinbah Valley Road.
You're better of speaking with a Brisbane local for things to do and see around Brisbane city.
Cheers,
