I like the idea of topics, but you do need to be flexible, as it's much more likely you may find someone before you has chosen the song you were going to do. But if everyone had done their homework, it can make for a very interesting session.

I also like an idea that I once came across at a singaround: a "chain" theme, in which each song has to be linked in some way with the one that went before it. That really keeps everyone on their toes and prevents the lazy singers from falling back on the same old songs every time.

Marje