Some tips, from experience:
> Just pick a date and destination. By doing this you make the meetup choice immediate for the potential attendees and force them to decide to come. It's useless to paralyse yourself in finding the perfect date/location, you'll never satisfy everyone.
> Have a couple of subjects prepared to talk about. One person 'hosts' the meeting and more or less prepares some subjects in case structured discussion is desired during the meeting
> The host does a small tour of the city for cultural purposes.
> Have dinner with eachother! This improves comradeship massively. If things are going swell you could expand things further to go out together. As long as it's an informal/casual activity should improve group bonds, really.
> Meet at a central trainstation for accesability. Move location to different places so more people are able to attend. Some will post their appearence (clothing/hair/etc) for others to recognise. In this way a group comes together. This way newfags are able to view this group from a distance anonymously, diminishing the investment for them. If you don't like what you see, you don't join in. Which nobody has done yet.
> Make an open invite for other people to come. Set a date and time and just meet at the entrance of the central trainstation. People will respond in the thread if they intend come. And then day before ask people to confirm if they intend come. So you know on how many you should wait.