I still think that an open public place, like a park or somewhere else where you can have a barbecue or something (located in a busy place like Melbourne) would be more appropriate for something like this.
Sure, everyone on here might be trustworthy but face it - this is strangers meeting up with other strangers from a public forum. I don't think it's worth the risk of compromising the safety of anybody by rocking up to a less busy and open place like a gaming centre or some random small great restaurant in an obscure place etc.
A more public place would also be a bit easier to locate as well. Museums, aquariums, cinemas etc. would be nice, but some of that stuff can be a bit costly (I'd say if you're going for stuff that people are going to pay for, keep the minimum money required to participate be less than $15 or some other fairly low amount).