There's a good walking track that leads from williamstown along the Yarra, then along the Maribyrnong through footscray and out past Avondale heights. The train will get you to Wiliamstown, Newport or Footscray at one end and trams/buses at the Avondale Heights end. Or start at Avondale Heights and head north through Brimbank park, etc:
In the other direction, whilst you wont see much wildlife, Studley park in Kew should be pretty at this time of year, particularly around the studley park boathouse