It started unassumingly enough. No media campaign, just a brief mention in the newsletter that goes out to customers: Canlis brothers Mark and Brian would hide 50 vintage menus around the city, at locations significant to Seattle history or culture. Clues would be delivered by Twitter and Facebook.

Find a 1950 menu, win a dinner for two at 1950 prices.

It was just a sideshow in the celebration leading up to the restaurant’s 60th anniversary, say Mark and Brian, but the brothers got a hint of how big it would get on the first day. When the first clue went out, “Squash a Beetle in your hand,” they sat nervously at the destination–the Volkswagen-crushing troll statue under the Aurora Bridge in Fremont–wondering if anyone would show. Then, they say, people started streaming towards them from all directions.

It’s only gained speed since then. Three friends have started a blog called Menu: Impossible about their quests to get one. Visitors swarm the Canlis Facebook page for hints of when the next clue will drop. The menu found at Bruce Lee’s grave involved cars peeling into a cemetery and a footrace to the marker. The staff at Canlis watched in delight through a telescope from the restaurant as people swarmed Gas Works Park for the menu hidden there.

Then, a whole new level: When some of the first winners showed up at Canlis to claim their prize they received a letter, explaining the menu hunt was only part one of the competition. Here’s the second part: The fifty winners are invited to take part in another hunt on New Year’s Eve 2010. The high stakes prize: a golden ticket good for a free yearly Canlis dinner for two, for life.

The twist? The winner has to give it away, every year. The Canlis brothers envisioned the prize as a way to reward those people doing important charitable work in Seattle.

As I write this, Brian and Mark Canlis—plus Canlis chef Jason Franey—are on their way to New York to tape an episode of the Martha Stewart show. They say that marketing people from all over the country are calling them, telling them the game is “genius.” Winner Adrienne Moore, who found a menu floating in the water at the halfway mark of a crew race course, agrees. She’d never eaten at Canlis before. Her friend tipped her off to the contest when it was re-tweeted by someone with whom she’d done bike messenger scavenger hunts. While she was excited to win the meal, she acknowledges that Canlis wasn’t on her radar before. But now, she says, “I’m sure it’ll be an option when I’m thinking about where to go in the future. Brilliant.”

Brilliant indeed.