George Clooney and his wife Amal.

The 'Monuments Men' star and wife Amal, 37, have been staying at their Italian residence, and were joined by around 10 pals including Julia Roberts and her husband Daniel Moder for the holiday.

According to E! News, the group sailed around Lake Como and enjoyed a late-night fireworks display.

Meanwhile, the 54-year-old actor stunned fans during Toto's concert in Milan on Thursday when he stepped out onto the stage to join pal Steve Lukather and his band mates during a rendition of the group's most famous hit 'Africa'.

A source told E! News: "He just appeared out of nowhere. Nobody realized for a while who it was and then everyone around started screaming and shouting. They went crazy when they realized it was George."

Another onlooker said the 'Gravity' star "looked dapper, very smooth and above all, relaxed and happy" during the show.

On Wednesday, George and Amal joined Robert De Niro and his wife Grace Hightower for dinner as the veteran actor and his spouse are holidaying in a nearby resort.

The 'Tomorrowland' actor arrived in Italy a few days before his British wife and quickly settled into vacation mode.

A source told People magazine: "George seems to be slipping into his summer routine."