THE bucket-list of “things to do before you reach the age of forty” has recently expanded. Now it includes an item that used to be consigned to a retirement list. Take a cruise. For cruises, particularly those to relatively unexplored locations, or on big party ships, or even in the company of fellow nerds who share your particular geek-interest, have become cool. Of course, there are still plenty of people in their golden years who treat them like floating retirement homes – the average age of British cruisers is still 55 – but a shift is happening. Cruises are being designed for people who want to party, have a genuine adventure, or just hang out with like-minds.

Cruise 18-30

If you’re concerned that if you book yourself on a cruise you may end up the only youngster among people twice your age, then, worry not, there are already particular cruise companies with the Millennial in mind. Last year when U by Uniworld launched its river cruises, it made it clear that it was pitching itself at the young by explicitly giving an upper age limit of 45. Now, however, it’s dropped the restriction, so even the young at heart can get on board and enjoy one of their cruises along the waterways of Europe - up the Danube and down the Rhine. Expect mixology classes, silent discos and local celebrity DJ parties.

Raving on the waves

Recent years have seen the rise of the festi-cruise, a kind of mini T-in-the-Park-on-the-Med. The formula is simple – one big music party on a luxury liner around the Mediterranean. The Ark made its maiden voyage last year, with the Ministry of Sound, Elrow and Deep House Amsterdam delivering the on board music, and was such a hit that it’s back again, with two cruises in August and September. It’s also not just about the DJs and partying – there are also the guest chefs, the yoga instructors, and, um ... the climbing walls. In short everything you might expect from a festival fused with VIP luxury. Why camp for your music – or even glamp – when you can cruise?

The Fan Cruise

One voyage, many stars, is the slogan for a Star Trek cruise which basically provides a trip on board the Seaship Enterprise, sorry Norwegian Jade, with a bunch of other Trekkies and a bunch of the actual cast of the show, including Jason Isaacs, Will Wheaton and Marina Sirtis, as well as musician Thomas Dolby. It's sounds mad, but then it is in the Caribbean and the sun has probably gone to everyone's head. This is for those who feel they’ve done the Star Trek conventions and need to take their fandom to a whole other level. But if you’re hoping to be beamed up for the 2019 cruise, you’d better get in their fast – since berths sell out rapidly. Though, of course, if Star Trek’s not your thing, there are plenty of other sailings to delight your inner nerd - there's Disney Cruises, the Star Wars Day At Sea experience, which includes surprise on-board appearances of characters and Jedi knight training, or the Doctor Who Sci-fi Sea Cruise.

Drifting down the Danube

The wetlands of the Danube delta, where the river meets the Black sea are a UNESCO-listed reserve, boasting the third largest biodiversity in the world. Its 5,500 species of flora and fauna are only beaten by the Great Barrier Reef and the Galapagos islands. A European river boat cruise, run by Fred Olsen, can take you right there, put you in a small safari boat and get you up close to the pelicans and red-breasted geese. It can be seen as a sign of the growing popularity of river cruises that the company is now pitching them at the fickle British market.

Polar regions

The coolest cruises don’t just head for the sun. Some of them sail for the chill north and the northern lights. Among them is Hurtigruten, which offers cruises that don’t entirely break the bank. Their Northern Lights Promise trip even offers a deal where if you don’t get to see the northern lights, then you get another holiday for free. There are also flight connections from Glasgow for their Classic Round Voyage which takes in Norway’s entire coastal route, sailing past more than 100 fjords, crossing the Arctic Circle, and stopping off for land-based excursions on the way. Hurtigruten also offer tours to Antarctica.

The Party Cruise

Born to party? There are cruises that are built mostly around that. Among them are the giant floating fun palaces run by MSC, one of the world's leading cruise ship owners. Its newest ship MSC Meraviglia, which was launched last year, has an 80m promenade of bars and clubs and is covered by an LED dome which creates an atmospheric digital sky. Midnight parties happen every night and the boat also hosts a Cirque du Soleil at sea performance, staged in a purpose built lounge.

Galapagos Adventures

Partying isn’t everything, though. Some of the most stunning cruises are quite sciencey and pitched at the nature nerd who really wants a bit of adventure and to get up close to some of the most fascinating nature sights on the planet. For them, there is a wide range of cruises that are less about the bars and more about the resident experts, lecturers, and guides. Among them are the conservation-focused, naturalist-led Lindblad Expeditions-National Geographic trips to the Galapagos islands, which include kayak expeditions, paddleboarding and snorkelling sessions. G Adventures’ also offers a cruise, endorsed by primatologist Dr Jane Goodall, which delivers the chance to see wild giant tortoises and iguanas.

Eco explorers

One of the problems with cruises is they have a reputation for being phenomenally bad for the environment. German conservation group, Nabu, claimed that European cruise ships were spewing out 3,500 times more sulphur-dioxide than land-based vehicles. For the eco-conscious, there are still cruise possibilities, though. Peregrine Adventures, for instance, runs low impact, carbon-neutral trips in small boats which tour Iceland, Croatia, Vietnam, Portugal, Spain, and Senegal. They have even banned single-use plastics like straws and water bottles. Peregrine Adventures achieve their neutrality by offsetting into renewable energy projects.

River Safari

You don’t have to travel by land to see Africa’s landscapes and wildlife. Croisieurope offer a bijou cruise experience, on board the 16 guest RV African Dream, which follows the Chobe and Zambezi rivers, with stop-offs for land safaris, and a visit to the Victoria Falls. Along the way travellers will see pods of hippo splash in the shallows, fish eagles patrol the river and buffalo and zebra gathering at the water’s edge.

Amazon adventures

One of the best ways of seeing the Amazon rainforest is via the river itself. One such cruise is offered by the Delfin III, which takes travellers to see not only the wildlife of Peru, but the Inca ruins of Macchu Pichhu. Along the way, guests participate in shore visits and explorations by river skiff, and get up close to dolphins, or which there are two types: the grey river dolphin and the pink river dolphin.

Tall ship yoga

Ditch modern mega-ships for the more romantic experience of sailing on a tall ship. The Royal Clipper is the world’s largest fully-rigged sailing ship, and Star Clippers, which runs it, offers a range of cruises and packages on board. Among them are tours of the Caribbean, celebrity chef trips, or a Mediterranean yoga cruise. You can get into your cobra pose, while sitting on a sun-basked teak deck, under billowing sails.

A week on an ice-breaker

If you think that cruise to the North Pole we mentioned sounds a bit tame, try the polar excursion run by Noble Caledonia aboard the 50 Years Of Victory, a nuclear-powered ice-breaker in June this year. The trip involves forging through the Arctic ice-pack, taking a helicopter ride for an aerial view, lectures, and disembarking for a glass of champagne on the pole itself. It will set you back, however, £21,495.

Boating to the races

One for those who love their horses, as well as their boats. In the Deep South of the United States, there’s a paddle-boat cruise that takes you down the Ohio river, from Louisville to Cincinnati, with an option to watch the world-famous Kentucky Derby. The eight-day Derby cruise is run by the American Queen Steamboat Company, and includes an optional VIP package with access to the jockey meet-and-greets.

Knitting Cruises

Okay, so this one does sound like it’s probably for the older demographic. But knitting is no longer just for grannies in armchairs. It’s cool. Millennials do it. They do it in the back rooms of trendy bars, while discussing their favourite yarns and Tinder profiles, or at home, while "knitflixing". So, the kind of trip run by Craft Cruises, in which guests get to click needles while voyaging between the ports of Scotland, Ireland and Denmark, taking in the Danish fjords and Scottish coastlines, ought to be one for the true trendsetter. Stitchtopia also run a Scottish Isles cruise to the Faroes knitting cruise.

The Sacred Ganges

River travel, as they say, is all about slow travel. And the Ganges is the ultimate place to go with that flow. Jeannine Wilson reviewed such a cruise, run by G Adventures, from Kolkata to Farakka, for the website Cruise Critic, and observed that there was no such thing as a schedule. “Daily programmes are variously influenced by the ebb and flow of the tidal Ganges, weather conditions and spontaneous stops, which all add to the sense of adventure for open-minded clients.”