The Istana's lush green lawns were transformed into a playground for 25,778 people at the May Day open house yesterday. Children and parents launched water rockets across the grass, while some tried out a remote-controlled miniature Mars Rover on a simulated terrain.

Solar telescopes, binoculars and glasses were also on hand for visitors to observe the sun, all part of a project called Journey to Space organised by Science Centre Singapore, with support from the Lifelong Learning Council and SkillsFuture Singapore.

It aimed to engage children - and curious adults - to help them understand concepts related to space and science, in part by using observation, technology and hands-on activities.

A model of the presidential residence created with 200 Lego bricks was also unveiled in commemoration of the Istana's 150th anniversary this year. It was commissioned by the President's Office and designed by a Lego certified professional.

President Halimah Yacob said after the launch: "Today is Labour Day - a day where we celebrate the achievements of workers. I am glad to have met many Singaporeans at the Istana open house who made a special effort to enjoy the day out on the Istana grounds with their families.

"I was also happy to launch the Istana 150 commemorative model of the Istana building this morning. The Istana is an iconic landmark in Singapore, having been an important part of our history and national identity," she added.

"I hope Singaporeans will appreciate this special and unique keepsake and, through it, look back on our journey together as a nation with pride."

Only 500 sets of the limited-edition models were available for visitors who made a minimum donation of $70 to the President's Challenge, an annual community outreach and fund-raising campaign. The sets were fully redeemed within two hours.

Financial consultant Mustafa Motiwalla, 35, was at the event with his wife and two children. He said his four-year-old son had recently learnt about space, the sun and the moon in pre-school and was excited to try out the booth activities.

He added: "It's not often you get to go to your President's house, so it's a novel experience... It's good to be on the grounds and get a feel of what it is like."

A 35-year-old dentist who wanted to be known only as Mrs Lin also said the science booths were a good experience for children.

She said her son and daughter, aged five and three respectively, tried out the rocket launch and also built paper helicopters.

"I wanted to expose them to science, experiments and daily things that happen around them. I think it was fun for the kids," said Mrs Lin.