Overseas visitors and locals alike have shown enormous interest in sites connected to the September 11 attacks on the twin towers.

More than 700,000 people from all 50 US states and 131 countries around the world have been to the National September 11 Museum since it opened May 21. More have come from New York than any other state, but the museum also hosts so many international tourists you can't identify all the languages being spoken.

In addition, nearly 15 million people have visited the September 11 Memorial since it opened three years ago on the footprints of the twin towers. That's a million more a year than visit the Statue of Liberty.

And yet the very idea of 9/11 tourism remains controversial. Some New Yorkers are still so traumatised they've avoided the area. Others think 9/11 tourism is unseemly, however respectful the intent.

Indeed, the memorial plaza could now be mistaken for a leafy urban park, with visitors taking smiling selfies or leaning on bronze parapets that bear the names of the dead. And yet, you could argue that recreating a sense of normalcy downtown is part of the 9/11 story, too.

"There's this tension between a nice park where you can come out and have your lunch, but you might be sitting next to a family member paying respects to a loved one," said Brenda Berkman, a retired fire lieutenant who was there on September 11, 2001, and worked on the recovery effort. Berkman now leads guided tours from the 9/11 Tribute Center.

For those who do want to visit, pay respects or learn more about the events of 13 years ago, here are some options.

9/11 MEMORIAL:

The memorial's waterfalls and twin reflecting pools are set deep in the twin towers' footprints. The pools are surrounded by panels inscribed with the names of the nearly 3000 people who perished in the attacks, including those who died at the site, on the planes and at the Pentagon. Also listed are six who died in the 1993 Trade Center bombing.

Hundreds of oak trees line the plaza, but be sure to find a tall, callery pear tree called the Survivor Tree, grown from a stump found in the rubble of the fallen towers.

The memorial is free and open daily, 7.30am-9pm; http://www.911memorial.org/visit-911-memorial . Visitor passes and security checks are no longer required.

9/11 MEMORIAL MUSEUM:

The museum tells the definitive story of the World Trade Center, from construction to destruction to rebirth. Artifacts large and small include tower beams, the Survivors' Stairs used by hundreds to escape, a wrecked firetruck and shoes worn by a photojournalist injured that day. You'll hear phone messages left by people trapped in the towers, and if you dare peek behind an exhibit labelled "disturbing," you'll see photos of those who jumped. Numerous videos and photos show Lower Manhattan before the attacks, as the planes hit, while the towers burned, and after they fell as enormous debris clouds covered downtown.

Tissue boxes around the galleries testify to the museum's visceral, emotional impact. But some of the simplest exhibits are among the most memorable. A wall of blue squares represents an artist's effort to remember the sky colour that day. And a massive wall bears this quote from Virgil: "NO DAY SHALL ERASE YOU FROM THE MEMORY OF TIME."

The museum is open 9am-9pm, with the last entry at 7pm through September 21 (after September 21, 7pm closing). Get timed tickets online in advance to avoid long waits. Admission, $24; http://911memorial.org.

9/11 TRIBUTE CENTER AND TOURS:

In contrast to the large, formal exhibits of the Memorial Museum, the 9/11 Tribute Center is a small, intimate, low-key place. The walls are covered with victims' photos and missing posters; handmade paper cranes – a symbol of peace – hang over a stairwell. It's like looking through someone's scrapbook and sharing memories.

The Tribute Center also offers terrific guided tours of the memorial led by individuals with a connection to September 11th – first responders, survivors, those who lost loved ones. The tours offer personal memories and insights as well as an informed appreciation of the memorial's design and symbolism. "We try and bring you to that day based on what we witnessed," Berkman said.

The Tribute Center at 120 Liberty St is open Monday-Saturday, 10am-6pm; Sundays 10am-5pm, with walking tours at 11am, noon, 1pm, 2pm and 3pm Center admission, $15; tours, $10; combined admission plus tour, $20; http://tributewtc.org .

ONE WORLD TRADE CENTER:

The observatory at One World Trade Center won't open until next year. But the gleaming skyscraper itself, with its trademark spire and graceful angles, can be seen from all over the city.

APPS:

Two free, official apps can help you explore the memorial and museum in person or virtually: the 9/11 Memorial Guide and 9/11 Memorial Audio Guide.

GETTING THERE:

Access the 9/11 Museum via Liberty and Greenwich streets, or Fulton and West streets. Nearby subway stations include Chambers and Fulton street, which are served by many subway lines. The E train has a World Trade Center stop