About five years ago, the Hard Rock Hotel Las Vegas realized it had a bookings issue. Guests were arriving for the weekend on Friday but leaving for home on Sunday, making Sunday evenings around the rock-music-themed lodging and casino kind of a snooze.

Not to mention a cost sink, because beverage sales fell off along with room bookings. The hotel realized it needed to figure out a way to keep some guests over Sunday night and to bolster those revenues.

“So they decided to throw a big pool party on Sunday nights and to make it crazy,” says Jay Schwartz, president of IdeaWork, a Santa Barbara CA-based interactive marketing agency. Schwartz’s group was already handling online marketing for the Vegas hotel, including designing its main Web site; so the Hard rock turned online promotion of the weekend pool party over to them.

Five years later, and the Sunday poolside bash at the Vegas Hard Rock—now named “Rehab”—is a big success. The hotel has an award-winning pool surrounded by cabanas and offering swim-up blackjack tables. Now on Sundays it brings in a DJ, offers bottle service at the cabanas and puts out the invitation to the locals to throw on a bikini and come on over.

This year IdeaWork has developed a microsite for the weekly party, www.rehablv.com, and a viral application that encourages site visitors to spend more time on the site and to e-mail others.

The backdrop for the site is an online version of the Hard Rock Las Vegas’ actual pool setting; the theme is basically young women in small bikinis and high heels. The site offers up information about the coming Sunday’s bash, a gallery of candid stills from the season’s previous parties that visitors can rate and upload to their MySpace or Facebook pages, and an application to enter the “Ms. Rehab” contest.

Visitors can also play a game of girl-girl online pool chicken, picking a team of one bikini girl riding on a large dude’s shoulders, then pitting them against other visitors’ teams, using keyboard commands to punch and kick their opponents underwater. To play the game, users need to submit their ZIP code, age and e-mail address.

“It’s a fun, silly thing to get people’s attention and keep them engaged with the brand when they’re at their office in Chicago or New York,” says Schwartz. The Rehab widget does something of the same thing, offering a new party photo daily along with information about the performers and other entertainment for the upcoming event, along with Vegas weather and links to lodging specials at the Hard Rock Las Vegas.

The photo gallery is one key to building traffic to the site and generating longer stays, says Schwartz, particularly in combination with the viral marketing tools. “You can see spikes in the send-to-a-friend traffic from the photo galleries,” he says. “The two main spikes are on Monday and Thursday. On Thursday they’re sending pictures of last week’s party to get a bunch of their friends interested in going. On Monday, they’re sending photos to their friends saying, ‘Remember that girl I told you I met? Here she is!’”

Even though IdeaWork’s been doing the Rehab promotion for half a decade, Schwartz says they haven’t settled into a repeatable formula. “We learn from the previous year and try to take it up a notch every time,” he says. This year the desktop widget is new, the game is different, and the microsite has a greater degree of interactivity than before.

And the metrics for the Rehab microsite have stayed strong. “Without getting too specific, we’re getting several thousand unique visits a day consistently,” Schwartz says. “Length of stay averages in the two-minute-plus range.” Most of that traffic is coming through viral marketing via e-mail and the social networks, because the site’s only real referrer is the main Hard Rock Las Vegas site; the hotel is doing little to market Rehab online through display ads or any search other than organic.

“People who go to Rehab on Sunday just know enough to check the site on Monday to see photos,” Schwartz says.