On a cold Saturday morning in January a year ago, two dozen youth looked at an overgrown, empty lot in Pittsburgh’s Allentown neighborhood and envisioned the future.

Where weeds, climbing vines and piles of debris concealed the small hillside wedged between a warehouse and the Brashear Association’s Allentown Learning and Engagement Center (ALEC), the children told Amber Rooke, the organization’s education coordinator at the time, that they wanted to plant flowers and vegetables. They also wanted to build a tree house, pizzeria and roller coaster.

The youth presented their ideas in the form of drawings to Ian Brown of GTECH — Growth Through Technology + Community Health — an agency that transforms vacant lots (Pittsburgh has more than 27,000 of them) into useful community spaces, including play areas called Green Playces. Mr. Brown later gave the children a menu of options from their suggestions to vote on, and they decided to install a rain garden and plant vegetable garden beds, to complement another vegetable garden created the previous summer.

They also made compromises: Since the lot did not have a made compromises: Since the lot did not have a sizable tree for a tree house, a large, ground-level “bird’s nest” made of tree branches would provide a nook for reading and quiet time. The pizzeria became a round garden bed with pizza-slice–shaped dividers and vegetables that could serve as ingredients for a healthy pizza, such as peppers, tomatoes, basil and onion. And a new slide would create the whoosh of a coaster.