Someone said they didn't like themed singarounds: this is something we do once a month a one club I go to, and usually works well. Inevitably someone will turn up not knowing it was the theme week, so we then invent the most tenuous and devious connections between what they choose to sing and the theme - all part of the fun and hilarity. It also encourages people to look outside their usual repertoire, with sometimes stunning results.
On discipline and etiquette, probably also worth making a statement about letting folk say if they wish to be strictly UNaccompanied, or whether they are happy for all and sundry to join in on whatever battery of voice and instruments turn up. I saw one a cappella trad singer doing an amazing job of putting would-be accompanists off by liberal use pregnant pauses and extreme rubato - and it just about worked!