Can't go past the bay! Park somewhere at Elwood, and head down towards Brighton. I used to do it for two hours every day, with pram and dog (and get paid!)

ETA - Also up the Albert Park end there's some good park and walk spots.

The Yarra Walk - you could probably park at Victoria Gardens or somewhere round there and then go down by the river, that's a stunning part of the world.
Also, up the back of Kew, the Eastern Freeway trail is wide and long and lots ot look at.