Every Saturday and Sunday at various Bunnings stores across the state, children over 5 can participate in free DIY activities. Times and projects vary, so it's important to check what is happening at your local store. You can also find other DIY projects that you can do at home with the kids on their website.
If you are there outside of the hours that the classes are ran- you can enjoy their cafe and in store playground. And you can pick up all of those bits and pieces that you might need around the house too.... ;)