A fun free thing to do when you need some spare time to kill is walk through Hosier Lane which is located near Flinders Street Station. It is a great cultural thing to see in Melbourne and can change over time so you can do it more then once. It might be nice date idea to grab a simple bite to eat beforehand and take it there to eat as you look at the art. I recommended it as a photo shot location as well. Definitely bring your camera to snap some nice shots. I would try to go during the morning during the week to avoid the busy crowds that can sometimes occur. This is one of the most popular graffiti art lane ways in Melbourne. Be sure to check some of the other ones out too.