sunshine coast
*Underwater world is at mooloolaba
*The ginger factory - buderim
*The Big Pineapple
*Aussie World / Ettamogah Pub - on the highway, just south of sunshine coast
*Obviously, Australia Zoo
*Maleny / Montville - sunshine coast hinterland

Brisbane
*Southbank - incl. museums etc
*Mt cootha botanic gardens and planetarium - we take bread to feed the ducks in the gardens, the kids love it.
*Mt cootha look out - go to the top, there's cafes, great to have an icecream and look around
*boat ride on the river cat in the city

Gold Coast
*i'd choose seaworld over all the others for a 3 yo, sesame street beach is great
*surfers paradise - there's an 'aquaduck' tour which is fun for the kids, you'll find brochures etc all over surfers paradise
*cheap - monorail from the casino around the shops, costs like $2 and the kids love it

that's about all i can think of at the moment, that we've actually done, hope they help!
Have a great trip xoxo