Outdoor learning is helping to improve the behaviour, fitness and well-being of under-fives, but schools are still not fully realising its potential, according to a report published this week.
Estyn found that most schools are making at least adequate use of their outdoor spaces to improve learning in the play-led foundation phase and that most children are highly engaged and enjoy the experience, particularly boys.
Overall, children's outdoor learning experiences were judged to be good or better in two-thirds of the sessions observed by inspectors.
However, the report says teachers and support staff are not using the outdoors well enough to develop skills in reading, writing, the Welsh language and creativity.
It also says a few schools are still sceptical about the benefits, and do not make best use of their environment as a result.
Since 2008, the foundation phase has been gradually rolled out to different age groups and, from this month, it will be taught to all three to seven-year-olds in local authority schools, church schools and nurseries.
Chief inspector Ann Keane said: "Learning outdoors is an important part of the foundation phase.
"We have seen many rich and varied outdoor environments in schools and settings such as areas for bird-watching, growing vegetables and learning about the weather and wildlife.
"Children generally take pleasure in learning outdoors and enjoy the whole experience. Teachers and support staff recognise the value of learning outdoors.
"However, practitioners need be more creative and confident in using the outdoors to develop children's knowledge and skills in all the areas of learning."
Among its eight recommendations, the report says schools should provide more opportunities for children to develop and practise their reading and writing skills outdoors, and plan outdoor learning as well as they plan their indoor provision.
It also says local authorities should provide training for heads and managers to help them identify good standards.
David Evans, associate adviser for outdoor education at the Education and School Improvement Service (ESIS), said a lot has been achieved in a short time.
"This is a relatively new concept in Wales that is a different experience for practitioners as well as children, and it is obviously going to take time for the changes to bed in and for teaching methods to alter," he said.
"While most schools are doing a good job there must be support for those that aren't and encouragement for those that are."
Mr Evans said many practitioners are often held back by fear of something going wrong.
"We need to get the message out that it's OK to let children play outdoors, and we shouldn't be scared if they fall over and graze their knees - it's part of the experience."
Anna Brychan, director of heads' union NAHT Cymru, called the report "encouraging".
She said: "The outdoor classroom is one of the most innovative aspects of the foundation phase and it is obviously having a very positive effect on children's learning experience. The fact that it is particularly beneficial for boys is extremely good news.
"Now we need the time and resource to make sure we can train all our practitioners to aim higher for the benefit of all children."