There's heaps if you look around (seriously, it would take you hours if not days to read through them all).
You could try things like opencourseware (sorta uni style lectures) or places that offer short courses like CAE.
I'm doing a intro course to a martial art right now and i'm really finding to be a blast. Just narrow it down to things you've always wanted to do or are interested in. Maybe pick up something you will actually keep going long term as well (no use taking up running for only 2 weeks by in large). Alternatively, pick up something that designed to be totally completed in a short time (many CAE courses etc).
Also consider things you might need to do around the house or your life in general as well, things you've never gotten around to.
Melbourne is a wonderful city, there's bundles of things to do (plenty free). How much of this city have you really seen? Have you ever been to the galleries (if thats your kind of thing) or laneway cafes before? Stuff like that. You really need not look far.