The soon-to-be groom, 31, who is engaged to Jessica Biel, is due back in the States Sunday after his extra-long (border-crossing) weekend with a posse of 25 pals.
A dressed-down Timberlake and his company, which included longtime pal Trace Ayala, arrived at the Mexican bachelor destination Friday.
After keeping it low-key with a planned game of golf Saturday, Timberlake's crew, who stayed seaside at the El Dorado Golf & Beach Club, were due to go out one last time that night before heading home.
What Happens In Vegas…But for Timberlake, the festivities began in – where else? – Las Vegas on Thursday with his partygoers all decked out in suits, a la the Rat Pack. His crew started off with dinner at Wynn Las Vegas & Encore Resort's Sinatra, an Italian restaurant featuring photographs of (decorated ladies' man) Frank Sinatra on the walls and films starring the late actor.
Following dinner, the group headed to Surrender Nightclub, where "JT was all smiles, though he stayed hidden" in a private outdoor cabana, a source tells PEOPLE.
Sipping 901 Tequila throughout the night, the Trouble with the Curve actor stayed tame (and clothed) as several of his buddies stripped down to their swim attire and jumped into the adjacent private pool overlooking the 60,000-sq. ft. nightclub.
"It was definitely JT's bachelor party," adds a source, who says multiple members of the group were overheard referencing the party as the reason they were in Las Vegas.
Though not a part of Timberlake's party, producer-rapper Timbaland was also at the club and went over to his old friend (and recurrent music collaborator) to send his wishes.
Later, the group made a late-night stop at Wynn Las Vegas's Tryst Nightclub.
Although the couple has not revealed a date for their big day, as of July, Biel, 30, insisted she wasn't in wedding-planning mode.
"I'm just enjoying being engaged," said Biel, whose plans to wed Timberlake made headlines in January. "You're in this romantic, ahhhhh, breathless moment that I'm just trying to extend as long as possible."