The 53-year-old actor, who became engaged to the British human rights lawyer last month, arrived at his fiancée's London home unannounced earlier this week and whisked her away on a trip to nearby Surrey, in southeast England, where he took her to visit the set of one of her favourite TV shows.
A source told The Sun newspaper: "George took Amal completely by surprise.
"He turned up at nearly midnight after jetting into Heathrow.
"George literally told Amal to grab some clothes before they jumped in a cab down to Surrey.
"She though it was so romantic, she loves how unpredictable he is. The set visit to Downton Abbey was the icing on the cake of a crazy few hours."
The couple are said to have indulged in a romantic stroll around the grounds of Highclere Castle, where the ITV period drama is set.
Meanwhile, 'The Monuments Men' star is expected to meet his future mother-in-law, who lives in nearby Buckinghamshire, while visiting the UK.
It's been reported that George will relocate to London once the pair have married so that Amal can continue her work as a barrister at the Doughty Street Chambers.