Given such a beautiful weather -sunny and cool - the outdoor is where you would want to be. Furthermore, enjoying nature is FREE ;)
Cycling to/from Port Melbourne -St. Kilda Beach is a good and easy route, even for families with small kids. There's a dedicated bike lane so you won't get in the way of pedestrians.
Ride or bring your bikes there; or if you haven't got one, you can rent them for a day from the bike hires (google bike hires melbourne). Or, buy cheap used bikes at Melbourne Gumtree (online classified ads).
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