Another vote for Macedon Ranges, you can drive to the top of Mt Macedon and there is a very brief (500m) paved path to the cross on the top which has a great view back to Melbourne.
You could easily do Mt Macedon and Hanging Rock in one day, perhaps Mt Macedon in the morning, have a coffee at the cafe and then drive the 20mins to Hanging Rock and have a bbq/picnic. We have been to Hanging Rock twice and haven't done the walk to the top so it's still an enjoyable non walking place to visit and they have a lovely cafe and playground as well. Be warned that there is a $10 fee to exit the park.
I also highly recommend Kinglake, it has a beautiful playground for the kids with dry stone creek beds and fallen trees as seats, it's really natural and non-plastic.
Also in the Kinglake area is a national park with a fabulous outdoor bbq and a short but quite steep walk to the falls.
As you can see I love this area and as we live nearby we visit it regularly, my boys are 4.5 and 2 and I would really recommend all these places.