The bus leaves every 10-20 minutes and costs AUD24 for a return ticket. It takes approx 25 minutes each way, airport to Spencer st on the west edge of the MEL CBD. Taxi is about AUD43-48 to city and AUD40-44 back to airport. Traffic should be reasonably light that time of the evening.

There is enough time for a meal in the city if desired. South Bank across the river has many reasonable options, I'm sure others can be more specific.

Assuming QF29 you would need to be heading back to the Airport by 9:30pm at the latest to be sure of comfortably making it.