Mila, who has been dating Ashton for nearly two years, told Cosmopolitan how the couple like to spend time together.
"We have a thing called white T-shirt night," said Mila. "It's when you wear jeans and a white T-shirt and you go on a date."
"Nothing fancy," she added. "It's just something that you love to do, whether that's bowling or dinner or going to a cheese shop and have some wine or walking around."
"I think that's just the best thing to do," said Mila. "Sometimes it's just nice to stay home."
Mila, 30, and Ashton, 36, often keep a low profile as a couple.
For the Two and a Half Men star's birthday on 7 February, Mila threw him a private surprise party.
The Black Swan actress rented out one of Ashton's favourite restaurants in Hollywood, La Poubelle, to ring in his 36th birthday.
"It was very small," an insider told Us Weekly. "Ashton has been going there for year and years. It's one of his favourite spots, like home, so it was comfortable and they were well taken care of."
Mila, who gets on well with Ashton's friends and is "part of the crew," asked his pals to help plan the night.
"Ashton didn't want a huge thing but they got a good group together," added the source. "He's certainly changed from how he used to like to party years ago. This was more of a wholesome situation. They had a great time."