How about a ferry ride to French Island - hire some bikes and tour the island or do birdwatching or something like. Can do it all on public transport if liked. The train from Frankston to Stony Point is charming and you get to see the interior of the Mornington Peninsula away from the highways etc. The ferry also goes to Philip Island, and there’s stuff to see there and very good food available. I have to admit I really like French Island - the wildlife and birds are so very unafraid and very approachable to within photo distance.