Hello,
I dlike to spent some weeks in Australia in August but i don't know which city i will go.Cairns or Brisbane?


Brisbanes awesome. Great city, lots of fun. You can get to the Gold Coast on the train in less than an hour for a reasonable price.
Go to Cairns - for both weather and activities. Can be too cold to siwm at either the GOld Coast or Sunshine Coast in August, and Brisbane itself doesn't hold enough for the tourist. Nice enough mid-size city - just not a tourist destination - more a gateway to a lot of other attractions, but not ideal in August, as I said.
"Some weeks" suggests at least three so why not fly into Brisbane, stay a week then head by train or coach up to Cairns for the rest of the time. Fly into one and out the other. See 'em both and make up your own mind!

I've got to admit, I live in Brisbane. Good place to live but I wouldnt visit if i was from somewhere else. However, it does have a good music scene. Apparently it was named one of the top 5 cities in the world for bands or something along those lines after London and Berlin.

I agree with travelling up the coast from Brisbane to Cairns if you have the time. Stop off at Fraser Island, the whitsundays take around 10 days min. Cairns at that time is Great! Beaches, reef, rainforest, waterfalls...and yes, huge party atmosphere. Ideally min 4-5 days in Cairns.

OP, not IN Cairns, likewise not IN Brisbane. But you probably mean that anyway.
But choose Cairns as your hub/leaping off point.
I mean President Clinton chose Port Douglas (near Cairns to visit in Australia, not Brisbane). Gorbachev stayed up here last year also.
Things to do: hire a 4WD at least for some trips. The best way to get around. Go camping all over, except in Port Douglas.
Great Barrier Reef - preferably from port douglas but also from Cairns.
Cape Tribulation - rainforest walks and nice beaches, picturesque road,
Chillago - limestone caves - dry outback country and people etc.
Cooktown - via the coast road preferably. Needs a 4WD though.
Up to Laura for aboriginal cave paintings and bush experiences.
Down to Tully for white water rafting - won't be much water in the river though.
Boat over to Hinchinbrook Island or Dunk - camping on tropical islands
Visit coastal areas south of cairns such as mission beach for whatever goes on there.
Drive to Townsville, go to Magnetic Island and see the sights down there.
Out to Charters Towers for the gold fields.
Drive back to ATherton tablelands on inland route. Lots of things to see and do on the tablelands.
Markets at Port Douglas on a sunday morning, Yungaburra once a month.
Lava Tubes out at Mt Surprise.
Wildlife parks around Cairns.
Lots of stuff to do. Or you could just spend the whole time lying on a beach under palm tree.
There's plenty of seafood to eat but dining out is expensive in port Douglas. Not much night life.