If you visit Sydney Aquarium, in the same complex is a new attraction also, Sydney Wildlife World which opened late last year. Make sure you buy at least one postcard from each...please. 
If you take the Ferry to the beach at Manly you'll get to cruise the Harbour.
In Brissie for the day....if you've had enough animals in Sydney, get an all-day pass on the CityCat ferry system, you'll be able to catch from Dockside at Kangaroo Point and you can cruise the Brisbane River up and down stream. The CityCat stop at Bulimba is just a short walk from a nice cafe strip, and you can also get off at Southbank to see the Museum and Art Gallery precinct. Mt Coot-tha is the only high point in Brisbane...a hill really... but if you can get out there (about 10km from the city, tour or there may even be a bus) you'll see nice views and get the city in context.
But if you want the best wildlife day you could every want in Brisbane, contact me and I'll put you in touch with a mate that used to run wildlife tours here. He doesn't have his own bus anymore, but if he is available, you could hire a small car for the day, and he will drive you around to see some really excellent stuff for a reasonable fee. Fruit-bat colonies, wild kangaroos and koalas, etc great birding spots, plus you get to see the city as well, and he's a laugh a minute. Just PM me.
darren