Downtown Miami activist and entrepreneur Brad Knoefler was tired of looking out of his loft window at the five-acre pile of rubble across the street that was once the Miami Arena, so he undertook what must be the ultimate DIY project: He built a park atop the debris.

He didn’t just plunk down some sod, either.

Grand Central Park, which opens to the public Friday evening, boasts paths winding through a small forest of 250 trees — slash pines, oaks, royal palms, red maples, cypress and sweetgums — and a great sloping lawn, seeded though not quite yet covered with a green carpet of eco-friendly microclover.

Crowning it all is a 12-foot hill — built from the crushed concrete — with a panoramic view of downtown Miami and surrounding hardscrabble neighborhoods, which seem suddenly transmuted with promise.

At the park’s center is an oval plaza, lined up with the iconic Freedom Tower two blocks east, that will be key to its success. The plaza will host concerts, festivals and markets that Knoefler says will generate enough money to cover maintenance and operational costs while attracting life to Park West’s desolate streets.

“This was a wasteland,’’ the gangly, fast-talking Knoefler, 44 , said between sips of Gatorade as he directed a small crew watering the soil and still-rooting trees and steamrolling the dirt paths flat just two days before the opening.

“It’s already changing the local dynamics. It’s becoming a real neighborhood in a weird sort of way. In just a couple of months, I’ve been meeting all sorts of people I didn’t know, and I see people out walking by, which you never saw before.’’

Though privately built, owned and operated, the fenced park — with construction underwritten by $65,000 from Knoefler’s nonprofit neighborhood group and a $200,000 city grant — will be open to the public during daylight hours with few restrictions other than obtaining a free membership card.

Here’s the kicker: The park is meant to be temporary.

Knoefler’s group, the Omni/Park West Redevelopment Association, has a 36-month, $200,000-a-year lease with property owner Glenn Straub, the eccentric Palm Beach developer who bought the Miami Heat’s old home at auction from the city in 2004 and subsequently tore it down after finding no profitable use for the obsolete arena.

The park, Knoefler says, will not only provide Park West a badly needed lift, but also serve as a demonstration of what can be quickly accomplished in even the toughest urban neighborhoods with a bit of imagination, some money but not a lot, and, not least, cooperation between residents and a city that in the past failed to fulfill promises of improvement.

The old pink arena, built by the city to lure an NBA team and sold to taxpayers and residents of adjacent Overtown as a redevelopment catalyst, became instead a symbol of broken promises. Little redevelopment took place beyond two apartment complexes directly to the arena’s north, today isolated oases in a sea of vacant lots and crumbling infrastructure.

After the Heat moved to the American Airlines Arena across Biscayne Boulevard, Straub took control of the site but could not come up with a redevelopment plan and, amid a dispute with the demolition company, let the property — and a mountain of rubble — sit for years. Straub hopes to capitalize eventually on improving prospects for the neighborhood, which is being eyed for a convention center or casino.

Knoefler, meanwhile, had spent more than $1 million to renovate a century-old building across North Miami Avenue into a mix of live-work lofts and the successful Grand Central music venue. Inspired by the Tactical Urbanism movement, in which activists install temporary improvements in derelict urban zones to demonstrate their potential, he began pestering Straub and city officials.

“I got obsessed with it,’’ Knoefler, who lives in the loft building, recalled. “I mean, what is this, Beirut?’’

To make his point, Knoefler rented a bulldozer and began crushing and smoothing out the rubble piles, guerrilla-style. He also hired guys off the street to help him patch sidewalks and repaint curbs which he said the city was unwilling or unable to repair. He sent out mass emails accusing the city’s Southwest Overtown/Park West Community Redevelopment Agency of inaction.

When Straub agreed to consider a lease, Knoefler roped in New York City landscape architect and urban designer Walter Meyer, who grew up in South Florida and whose firm, Local Office, focuses a portion of its practice on pro-bono work.

To its credit, Knoelfer said, the CRA, and its initially skeptical chairman, City Commissioner Marc Sarnoff, came around, agreeing to the grant on the condition that Knoefler allow public access to the park. Construction began in October after months of permitting limbo.

Working for expenses only, Meyer and his partner, Jennifer Bolstad, developed a low-cost, low-maintenance plan that recreates, in compact form, South Florida’s coastal ecology — molding the rubble into a sloping topology that starts with wetlands and a cypress grove at ground level and rises up to a hardwood ridge, and is planted with both southern and northern tree species found naturally in the area. Ponds will capture rainwater for irrigation.

Knoefler seeded the sloping lawn with a mix of inexpensive yet wear-resistant microclover and hardy Seashore Papsalum grass, and topped the plaza not with asphalt but with PolyPavement, a nontoxic emulsion which mixes with soil to produce a concrete-hard surface.

On Heat game nights, Knoefler will offer cheap parking — $10 — on the plaza and food trucks for a tailgate party-like atmosphere, mainly with the idea of cementing the notion of the park as a center of neighborhood activity.

And that includes Overtown, Knoefler stresses. The park, he says, can help knit together resuscitating Biscayne Boulevard and downtown Miami with the long-excluded, historically black neighborhood. He wants Overtown kids to use the park, and has hired local residents to help build the space and installed two of them as caretakers. He’s also offering Overtown residents the chance to sell food, perhaps barbecue, in the park.

“I’m liking what I see,’’ said Clarence Woods, the CRA’s assistant director, who dropped by for an impromptu inspection. “It sure looks a whole lot better than a pile of rocks.’’