Every parent has watched bemused as excited kids toss aside gifts to play with the boxes instead. But what about when they ignore the shells on a tropical beach in favour of plastic bottle tops?

That was the puzzle for Ralph Groenheijde when he and his family visited Costa Rica a few years ago -- a trip that was to spark a passionate crusade to clean up the beaches back home in the Netherlands.

His then two-year-old son paid little heed to the shells, collecting instead dozens of brightly coloured bottle tops. Eventually they used them to create a giant sun mosaic on the sand, before depositing them in a bin.

It was to trigger Groenheijde's scheme not to just clean up the wide, sandy beaches skirting the coast of The Hague, but also to turn an unwanted "treasure trove" of trash into wacky works of art.

In a play on words, this summer's creations have been gathered in the new TrashUre Museum, where lost balls and multi-coloured plastic spades dangle like decorations from the ceiling.

Candy wrappers artfully adorn a rakish top hat tied with blue string, and a cascade of flipflops makes a rainbow floor sculpture.

A blue fisherman's net is hooped and hung as a dress on a dummy, while hundreds of cigarette butts spill from a giant box, offering a silent rebuke.

- Heroes and pirates -

Harnessing the power of social media, Groenheijde organised his first trash hunt in the Netherlands some three years ago and built a pirate ship in the sand with the finds.

"The moment that it was finished the kids came and started playing with it," Groenheijde, 44, a trained therapist and counsellor, told AFP.

"From that moment on I began calling the trash can a treasure chest, and from now on we are treasure hunters. We are pirates. We are saving animals. We are heros because of that."

When a friend offered him the free use of a building on the Scheveningen seafront, Groenheijde hit on the idea of a museum for the artworks.

Now he guides groups of adults and children daily on sorties, motivating them to clean up the environment, to get out and exercise and stretch their muscles as well as their imaginations.

This summer he set locals a 90-day challenge to scour the sands every day.

Since the end of June he calculates they have scooped up some 40 tonnes of garbage -- including 42 dirty nappies, 64 sanitary pads and 18 tampons, all of which are disposed of in bins.

These finds come despite efforts by local authorities. A 15-strong council crew heads out nightly using tractors and beach cleaners "digging and raking the waste from the sand" for 10 hours from the 11-kilometre (seven-mile) stretch of beach.

Annually 1.9 million euros ($2.1 million) are spent by The Hague to keep the beaches clean, and on a single busy day they can collect up to 100 square metres (about 1,000 square feet) of trash from over 400 large bins and the shoreline, a spokesman for the city said. That's equivalent to 20 king-size beds.

The dirtiest finds are also why Groenheijde calls his TrashUre hunters heroes for "daring to take care of the toilet" -- no-one wants to clean the toilet at home, but someone has to.

- Bags overflowing -

"I never expected to collect so much rubbish in 15 or 20 minutes. I was very surprised," said Jawad el-Woustati, who was among 20 young trainees from The Hague municipality sent to join Groenheijde on one expedition.

Giggling, the group initially turned up their noses at the task. But as Groenheijde divided them into groups and made it a challenge, bafflement gave way to enthusiasm. Soon they were pouncing on every bit of offending detritus and soon had three huge overflowing bags.

"Now I've seen what's on the beach, from now on I'm going to take a bag and collect everything to put in the bin," Woustati said.

The piece de resistance in the museum is a perfectly executed map of the world -- the continents are formed from some 30,000 cigarette butts and the seas dotted with bottle tops -- to represent all the plastics floating in the oceans.

"Unfortunately, there are so many people who don't use the rubbish bins," said Sophie Hermans, one of the group taking part that day.

"It's a very simple idea, and it would be so easy to do this around the world."