Plenty to do!!

as danni said
Southbank
crown
Eureka is great and you must do the edge!!
Docklands *amazing human body expo*
Museum
State library
Dandenong ranges
Phantom of the opera
melbourne show is sept
LOTs of winarys

depends if u have a car
Phillip island
moninton peninulsa
queenscliff

hope that helps
if u want any more ideas just ask