At least twice a month, volunteer nature guide Tham Pui San, 69, leads a group of about 20 people to a park or nature reserve.
They are not there just to enjoy the greenery or take photographs; they are there to paint.
Mr Tham, a retired engineer, has been running these paint-along sessions for over 13 years. He sees them as a way to share his twin passions for nature and art .
"I want to help people understand what they paint, know it better and appreciate it better, so that on a personal level they can do more for the environment around them," he told The Straits Times.
Nature first attracted his interest in 2002 when he was doing an oil painting on a wooden deck in Sungei Buloh Wetland Reserve and heard splashing below. When he looked under the deck, he saw five otters playing.
"I was so excited, I messaged my friends to ask them if they knew there were otters in Singapore."
This was before otters became more common, he quipped. Inspired, he went to the visitor centre and signed up to be trained as a nature guide.
He also volunteered to start paint-along sessions in the reserve.
He later started sessions at the Hort Park and Singapore Botanic Gardens. He spends one Sunday a month at Sungei Buloh, and a Saturday at either of the other two parks.
The free sessions, which last two or three hours, see a mix of middle-aged women and families with young children. Most are beginners.
Artwork by the Sungei Buloh group has been in two exhibitions at the nature reserve.
One member of the group, which has about 100 people now, is oil and gas trader Edwin Low, 57. He joined three years ago with his wife, Cecilia, and some friends.
He said that besides improving his art skills, the monthly sessions help him unwind.
"Painting amongst the lush trees and away from the hustle and bustle of city life is therapeutic and relaxing," he said.
For Mr Tham, volunteering has kept him active since he retired in 2007 after 42 years with Singapore Airlines' engineering department.
He learnt art on his own by experimenting and reading, prompted by graffiti drawings he saw as a child on the pillars of kampung houses in his Telok Blangah neighbourhood.
Even on the go, a pocket-size Kaweco pen and a water brush pen nestle in his wallet and daily activities - bus rides and hawker centre meals included - are captured in a pocket sketchbook .
While Singapore's nature scene may not be as varied as that of bigger countries, it is not dull, he said.
He once met some artists in Sungei Buloh who said it was boring. They weren't looking deep enough, he said.
"It's not the spectacular that attracts my attention, it's the changes that happen all year round," he said, giving the example of the Blind Your Eye tree whose leaves turn red and golden for only two weeks every year.
Nuances - the shades of green within the wash of green that first catches the eye - also attract him.
He said: "I've been painting Sungei Buloh for 13 years and I'm not short of things to do."