Given the ice crust encasing our streets right now, this might seem to be a strange moment to ponder what makes a good playground. A kid is lucky to gulp a few breaths of fresh air in the minutes between school and home. It feels as if it will be months before the slides are warm enough to touch again.

When the thaw does come, as it inevitably will, most children will race out to the kind of playgrounds that feature mass-produced plastic forts done up in circus-inspired colors and perched on spongy mats. Grateful to be liberated from their cabins, they'll climb, slide, and climb again. Then they will repeat that sequence over and over, while their bored parents stare at smartphones.

In these days of helicopter parenting and obese first-graders, any opportunity that kids get to run around outdoors certainly has to be applauded. And yet, psychologists and designers are starting to question the value of what they call "the playground in a box," the factory-made, cookie-cutter climbing frames that dominate schoolyards and parks across the country.

Susan G. Solomon, a Princeton-based architectural historian who has just completed a book about playgrounds called The Science of Play, is withering in her disdain for those nondesign designs. "Today's typical playgrounds are maintenance-free caged areas that emphasize safety more than critical thinking," she writes. Character-building adventure and imaginative play take a backseat to liability concerns. On top of that, she says, the equipment is expensive, making it hard for towns to keep up with the demand for play spaces.

Her criticism struck a nerve with me. There are whole neighborhoods in Philadelphia without a decent playground, and the closing of so many public schools has only made the situation worse. There have been several new city spaces, such as the lavish Herron Playground at Second and Reed, but they tend to follow the old playbook. What if the city ditched plans for new forts and opted instead for something rougher and more ad hoc, say, the playground equivalent of the pop-up beer gardens that have been so successful?

That would make play advocates such as Solomon and Robin Moore, founder of the Natural Learning Initiative, very happy. Not that they would ever use the word playground to describe their alternative vision. They prefer organized learning environments and loose play.

Their dislike of the off-the-shelf varieties springs from the belief that the designs are too prescriptive and controlling. Their fixed pathways prevent kids from setting their own play agendas, something they need to grow into self-sufficient, well-adjusted adults. "The equipment being produced today is so static," complains Solomon.

It wasn't always that way. After World War II, European architects turned out custom playgrounds that challenged kids both physically and intellectually. Inspired by their work, a few American architects, including Philadelphia's Louis Kahn, tried their hands at the form. But the movement didn't get very far. Playgrounds were a casualty of the breakdown of American cities in the '60s and '70s. As maintenance was deferred, they fell into ruin. By the time cities began to recover in the '90s, Solomon says, all that local officials wanted was equipment that was indestructible and vetted for safety.

Moore, a professor at North Carolina State University who has been studying children's play for 50 years, sees a connection between those designs and the increase in such childhood ailments as obesity, anxiety, and attention-deficit disorder. In the simple act of scrambling up the branches of a tree, a kid learns to monitor risk and deal with fear. But on most playgrounds, the climbing frames are lower than ever.

The concern about such controlled environments has sparked any number of popular books advocating less programming: Free Range Kids, 50 Dangerous Things (you should let your children do), Last Child in the Woods. All see our culture's fear of risk as worse than the occasional scraped knee or broken bone.

So what's the alternative to standard-issue playgrounds? Solomon envisions multipurpose, multigenerational urban parks that incorporate spaces where kids can take charge of their own play. Instead of a fixed bridge in a plastic fort, they would have to use their imagination to decide which objects could be converted to play equipment. Such a challenging play space also would include nooks where kids could temporarily escape the nervous gaze of their caregivers. There would be no fences, plenty of trees and bushes, and good seating.

Seating is key, says Moore. Parents need to feel comfortable, so they don't get antsy while the kids are enjoying themselves. Because the play equipment would be fused with the landscape design, these spaces wouldn't look anything like the playground as we know it.

Sister Cities Park, by Studio/Bryan Hanes, probably comes closest of any Philadelphia park to meeting the play advocates' goals. The star of the little Logan Square park is a man-made hill lushly planted with shrubs and trees, but there is also a "loose play" area where children can build a fort - or house - out of foam blocks. The park's cafe and shady seating also work for adults.

The Fairmount Park Conservancy has commissioned Hanes to design a variation for an older cohort, 7 to 14, in the Parkside neighborhood, near the Please Touch Museum. Along with a topographical play area, he's exploring an ice skating loop, rock-climbing wall, and an inground trampoline. But he also wants to make sure there is seating where teens can hang, "even if that means just sitting next to one another, playing video games."

Better still, why not have kids make their own play spaces? Rather than present kids with a finished design, Philadelphia's Alex Gilliam, founder of Public Workshop, enlists them to design and build pop-up parks. His idea of creative play involves the use of power tools, even by younger children. Obviously, the approach requires supervision, but his DIY spaces at Smith Memorial Playground and on a vacant lot in Camden have been a huge hit with their makers and users.

Besides being dirt cheap, the great thing about pop-up playgrounds is that kids don't have to wait till spring to start playing around with ideas.