Nice idea.
Publicise an interesting and topical lecture/talk/discussion with a particular science slant, host it in an informal setting and encourage the public to attend and contribute free of charge.

“Cafe Scientifique is a forum for debating science issues, not a shop window for science. We are committed to promoting public engagement with science and to making science accountable.”
From The Alchemist Cafe Dublin

Or join their mailing list to get notified of upcoming events (usually monthly).