Introducing the cruise line where over 45s are banned

"Hold the spray right there. Then move your hand back and forth. No, no, not all over the place; neater lines when you colour the wall, neater lines!"

I steadied my wrist and smothered the instinct to make a toddlerish scribble or scrawl 'Red Rum'. Then I sniggered: passers-by were chancing sideways glances at these English-speaking 'vandals' who didn’t seem sure how to hold a canister.

Yes, I was having a (perfectly legal) graffiti art lesson in a Parisian suburb, one of many shore activities that will form part of the world’s first cruise aimed at millennials.

U by Uniworld, a new brand from a line well established among the river-cruise fraternity, is reaching out to a new market; two ships called A and B, strictly for 21- to 45-year-olds, will sail for the first time next spring. They offer nine itineraries on the rivers of Europe, stopping in some of its most popular cities.

With a few months to go before the launch, the company invited a small group to Paris for a day on board 'The B'. A cruise virgin in my twenties, I represent just the target market it hopes to reach.

First impressions were good. The ship’s exterior wasn’t white; that would be conformist. It was a vampish, moody black with a lantern-speckled sun deck that twinkled at night. As I routinely suffer from seasickness on scuba-diving dinghies, I thought I wouldn’t like being on The B – but, to my relief, it felt like hanging out at a hotel on land.

Furnishings included funky egg stools, Ibiza-style island beds on the roof and more dangling filament lights than on a building site. There was even a polaroid selfie machine and a screen on the lobby wall showing passengers’ tweets, tagged #TravelForU.

I couldn’t help but feel that the main bar had been engineered primarily for Instagram: I loved the chequerboard-inspired interiors and blue-tinted lighting redolent of a 90s acid rave.

Seating so low-slung that my knees practically grazed my ears rounded off the posey vibe. My favourite space was the rooftop deck with its voluptuous purple sculptures and monochrome banquettes.

But photos of One Direction and Taylor Swift made me wonder whether B’s design gurus had got millenials mixed up with teenaged Generation Z. And the PVC booths in the main-deck lounge/bar were less “hip end of Shoreditch” more 'dodgy end of Soho'.

Street-food-inspired cuisine is served at communal tables. The lunch buffet I sampled – sprout salads and beetroot and squash bowls washed down with banana and spinach power smoothies and antioxidant blasts – was a masterclass in trending grub.

I had a taste of the graffiti tour in Paris, which will be on offer to cruisers

A sneak peek of the rooms revealed clever and elegant use of space, with matt red headboards, textured black-and-white wallpaper and full-length mirrors. The pricier rooms have balconies. I was genuinely heartened to see that cabin options included bunk-bed triples – great for groups of friends.

'Wine and paint' classes, yoga and silent discos are among the on-board activities. A mixology workshop at the bar began with a history of the word 'cocktail' (warning: vegans may get queasy from anecdotes about 17th-century rooster fights). We made several classics, from Aviators to Tom Collins. I came away with a bespoke drink honed to my tastes that should be easy to replicate at home.

Uniworld has cooked up an array of quirky excursions, from a club night in Amsterdam to kayaking in Bamberg, Germany (a Unesco World Heritage site). I found the Paris options alone impressive. The street-art tour taught me that the city’s urban artists love to draw cats and Space Invaders.

That it took place in the grittier streets of the Belleville suburb appealed to my liking for off-the-beaten-track travel. My housemate – a French expat – almost choked on his tea when I got home. "OMG. That area is super dodgy!"

Also on offer in Paris are retro motorcycle experiences; ideal for living out the Wallace and Gromit sidecar fantasy, or for those with a penchant for Forties fur-trimmed leather jackets, perhaps.

The surreal life on board the world's largest cruise ship

So would I – a 29-year-old – sign up for a U by Uniworld cruise? The price point is tempting: from £159 per person per day, including meals, some excursions and on-board activities – a good third less than I’ve spent on European city breaks. Trips are short enough (eight days) to feel low-commitment. And with enticements ranging from international DJs and market shopping with an on-board chef to five-star service, you know what: I just might secure my place.

U by Uniworld (0845 678 5656; ubyuniworld.com) offers eight-day departures from April 14, 2018. From £1,269pp aboard The A and from £1,519pp on The B, excluding flights.