Well, there are a lot of very different answers to that question: Frank Sinatra used to do crosswords; David Bowie was a compulsive chess player, as is Will Smith and GZA of Wu-Tang Clan; Owen Pallett likes to play Dungeons and Dragons; and David Lovering of Pixies practises sleight-of-hand magic tricks and takes a metal detector with him everywhere he goes.

Here's a further selection of part-time pastimes, some of which are even odder...

He may be a hip hop pioneer, turntable master and one-man vinyl revival (as discussed in this interview with Jo Whiley), but Grandmaster Flash's secret passion is the humble mug. He’s been picking them up as souvenirs of his travels for years and has amassed a collection of around 5,000 to date, which is kept in a climate-controlled and very secure lock-up. He told The Source: "The room temperature is correct and the place is insured."

According to Peter Ames Carlin's book Bruce, in the early-1970s the Boss would while away the boring hours between shows playing a souped up version of Monopoly with his New Jersey muso pals in which the Chance and Community Chest cards had been re-written. Neither history nor the down-home Bruce (as seen in this clip from the documentary The Ties That Bind) has recorded all of the forfeits and gifts they awarded each other, apart from the satirical "Race Riot" card, which, if selected, would burn down all one's houses and hotels.

Meanwhile, Kylie Minogue is apparently a demon Scrabble player. Her friend, the author Kathy Lette, told the Mirror: "We quite often have marathon Scrabble competitions. Kylie has the game down to a fine art - she knows how to score big and doesn't mess around. She pretty much always wins." And this fondness has even found its way into her lyrics, with the line "Desperately seeking someone willing to travel / You’re lost in conversation and useless at Scrabble" turning up in Your Disco Needs You.

The Muse frontman used to be a big fan of letterboxing - basically the Pokemon Go of the pre-Smartphone era. Players hide little weatherproof boxes in parks or open moorland and then send clues to their treasure's location to letterboxing websites, or to be printed in special catalogues. Matt admitted a fondness for the pastime in a conversation with NME: "I always go for walks in the countryside. The South Devon countryside... I used to go letterboxing, actually."

For someone as visually oriented as Miley Cyrus, it makes total sense that she would round off a hard day as a pop provocateur with a big pile of glitter, some cut-out card shapes and a hot glue gun. A quick peek at her (warning: generally quite adult-themed) Instagram feed reveals a riot of bright, sparkling and occasionally near-the-knuckle artistic ideas, including this sparkling tribute to Caitlyn Jenner, commissioned by Vanity Fair magazine.

When he's not scoring operas, making cartoon hip hop with Gorillaz, touring with Blur or forging links between indie rockers and world musicians, Damon likes nothing better than a spot of ping pong, as he told the NME: "Yeah, everywhere I go I have to have a table tennis table. I love it; it's a brilliant way to relax when you're working. I'm playing at the Lyon Festival and they wanted a slightly longer set, so I said, 'If you give us a table tennis table then you'll get a longer set.'"

Dave Peacock is a dependable pair of hands both as one half of cockney rockers Chas & Dave, and as a craftsman carving, painting and restoring old gypsy caravans to their plush former glory. He told Countryfile: "Gypsy wagons are in my blood. My grandparents were travellers and it was always there in the back of my mind… I'd work on things on my days off. I'd rush back from a gig and get stuck into a job but then have to be off on the road for six weeks before being able to get back to it."

Many seasoned metal veterans complain of a buzzing in their ears, but Steve Vai - master of the stinging rock instrumental - can put this down to his fondness for the humble bee. He told fans on his website: "Many of you know that I'm a hobby bee keeper. It's a simple and very rewarding hobby. The bees do all the work! Whenever I am working with the bees it's a closeness to nature that is rejuvenating and inspiring."

There are plenty of people in the public eye who hit the gym, but Ellie Goulding really hits the gym. She got into running when she was 18, and has since run several distance races, including a half marathon (she prepared this excellent BBC Music playlist to help joggers on their way). As this Instagram clip proves, she's training as a boxer, too. She told Glamour: "People used to laugh at me for my lifestyle because it wasn't cool, but fitness is booming now. I think people want to live a healthier life now."