You could try Shingle Inn? They're at Garden City or Carindale. They have some basic butter cakes that are decorated with smarties and the like that you can just buy from their display case. Anything fancier is a preorder. Their cakes are yummy too. I'll occasionally buy one of their patty cakes when I'm out shopping.
Second somewhere like Michels Patisserie.
Also you could try Crusty Devil Bakehouse. There's one on Stanley Rd shops Carina or the Woolworths shopping centre on Wynnum Rd at Cannon Hill. Have a look at their website, if you rang first thing in the morning you may be able to order a birthday cake if you just want basic icing/decorations.
I remember when we were looking for my sisters wedding cake we went to The Welsh Lady at Paddington. You might have to visit though to see what she has on offer, and it will probably be a bit more expensive.