'Maybe all the little things add up," says architect Mark Sanderson, standing on Logan Square in front of what will be the new Sister Cities Plaza, a cafe, pavilion, and garden designed by his firm, DIGSAU. This is surely the prayer of the decade: that in an age of shrunken budgets, a city such as Philadelphia can nevertheless reassert itself on the urban scene.

In the morning shadow of the Cathedral Basilica of SS. Peter and Paul, Sister Cities Plaza is indeed an ideal place to test the power of this prayer. The plaza is one section of one square on the long Benjamin Franklin Parkway, which itself is one stitch in the reknitting of Philadelphia. But if successful, the project will tell us a great deal about the potential of small interventions to transform the way we experience the city.

Like many public spaces in Philadelphia, Sister Cities Plaza has been a moribund place, heavy on meaningful gestures - in this case a monument to Florence, Italy, and Tel Aviv, Israel, two of Philadelphia's 10 sister cities - and light on things to do. Split by four and five high-speed traffic lanes, the square itself has long ceased to function as a unified space, its edges, including Sister Cities Plaza, taken over by homeless encampments.

In 2003, the Center City District began to think critically about how to re-stitch the Parkway, which opened in 1927 already compromised by the automobile. "We decided that a simple proposition would be to design nodes of activity," explains Paul Levy, the district's president and chief executive, "so that you would pass a cultural institution, a sculpture, a cafe, every two to three minutes." The district, masterful at fund-raising, raised enough from various sources - particularly the Pew Trusts, the William Penn Foundation, and the state, under the administration of former Gov. Ed Rendell - to install pedestrian and architectural lighting, revamp Logan Circle and Swann Memorial Fountain, upgrade sidewalks and curbs, and, in 2008, to build Cafe Crêt at 16th Street. Last year, the Fairmount Park Commission and the district designed interpretive signs for the vast collection of art and architecture.

In 20 years, many of the district's installations, including Cafe Crêt, have borrowed heavily from classical designs of a century ago. "In taste," says Levy, "I'm rather formally inclined." But Sister Cities marks a notable turn toward a more contemporary vision. "A realization came to me a few years ago, teaching students at Penn," says Levy, who for years has taught a course there called Downtown Development. "These kids are living in the same city I'm living in, but they're living in it differently," meaning the city ought to reflect this youthful informality and sociability, and the students' willingness to break boundaries and explore.

For Sister Cities Plaza, Levy hired young landscape architect Bryan Hanes, a member of the team that has designed half a dozen key public spaces across the city, including Swann Memorial Fountain and Independence Mall, to work with Sanderson and his DIGSAU partner, Jamie Unkefer, to make the withering space welcoming to families and children already drawn to the Parkway by the Academy of Natural Sciences, the Franklin Institute, and the art museums. Levy had always wanted a children's garden with a Luxembourg Gardens-style toy boat pond on the Parkway, but otherwise he required only that they build a cafe to be illuminated at night and a more revealing and engaging monument to Philadelphia's sister cities.

Demographic change certainly factors into Levy's recalibration. Center City, particularly, is growing younger and Levy wants the district to respond to market needs. Hanes and the DIGSAU partners are among a cadre of ambitious young designers with strong visions of the city's future. Meanwhile, our urban tastes and expectations have changed. "We use parks in totally different ways than the original planners envisioned," says Hanes.

"I think we see them now as outdoor living rooms," adds Sanderson. With Sister Cities, the district put no restrictions on architectural style. In fact, says Unkefer, "they let us go."

On the south side of the plaza, Hanes designed a clever multi-jet fountain that will represent Philadelphia and its sister cities as an urban constellation (with Philadelphia, of course, at its center). Nancy Gilboy, president and chief executive of the International Visitors Council, the agency that administers the Sister City program, believes the fountain "will bring the program alive," she says. "What's nice about Sister Cities is that we think of our partners as family. Families are welcoming, and that is what this is going to be, a welcoming place."

During the planning phase, Hanes spent a weekend in a cabin with his son Isaac, then 7, and one of the boy's friends. "They took it really seriously," Hanes says, and the boys drew pictures of what they would want to see in the park. The answer: a lot of rocks. "If they're big enough, you climb them. If they're small enough, you throw them," notes Hanes. The rest of the team spent time in the Wissahickon Gorge, imagining what it would mean to capture the ancient landscape and make it a playful part of the urban experience.

The result is a park, now under construction, that shifts the Parkway's immense scale, transforming it into a child's wonder world made of Wissahickon creek, cliff, and forest. The new plaza thus shrinks the space so that it feels more intimate and expands it all at once. "We're creating a universe here," explains Unkefer, who grew up just a few blocks away. A mountain stream at the north end of the site will feed the pond, which will meet a cafe whose low, green roof is meant to reflect the horizontal rock ledges of the Wissahickon. The cafe itself is a bright, forward-looking glass pavilion clad partly in Emerson limestone. The building is heated and cooled by a geothermal system.

Perhaps the youth movement is just the answer to Sanderson's prayer. "I used to go to London and see all this contemporary art and architecture and hate it," says Levy. "Then I realized they had the guts to be doing it, they had an orientation and willingness to do new things." We might then think of a small project such as Sister Cities not as patchwork on an old and tired city, but instead as a bolt of new material. Sister Cities Plaza - though sadly not the planned Mormon Temple immediately adjacent - may be a harbinger of the city yet to come.