The National Rhododendron Gardens in Olinda are spectacular to visit at any time of the year, though for me, autumn is a particularly special time of year to stroll around the gardens.
The gardens here are massive, you'll easily be able to clock up a few kilometres walking around the entire garden, but you can choose to see as much or as little of the garden as you please. My favourite section in the garden is towards the bottom of the garden (just after the Cherry Blossom Walk), it's where you'll find expansive open areas and a magnificent view towards the Yarra Ranges (as pictured below).
In my opinion this is also the best spot for a picnic lunch with children. We take a ball along and have a kick to kick before playing amongst the fallen autumn leaves.