CHICAGO — Barack Obama has seen presidential libraries. Sometimes they are a monument to the past, he said, standing on stage in an auditorium on Wednesday; a record of accomplishments. “And a little bit of an ego trip,” he added. “‘See what I did.’”

Mr. Obama and the former first lady, Michelle Obama, returned for a day to the city they call home to unveil renderings of the Obama Presidential Center, a modern, stone-and-glass complex on Lake Michigan that they said would be a different kind of presidential library. Speaking to several hundred people on the city’s South Side, Mr. Obama, who was tieless and in a jovial mood, said that the center could be “a transformational project for this community.”

“The main thing that Michelle and I contributed was just saying, ‘What is it that we want to see 10 years from now?’” he said, recounting his conversations with the architects who designed the center. “And we don’t want to see some big building that’s dead, and kids are getting dragged to it for a field trip. What we wanted was something that was alive, and that was a hub for the community and for the city and for the country.”

It was the second time in 10 days that Mr. Obama had come to Chicago, as he has begun to emerge in public after the end of his presidency. He spent the weeks after the January inauguration of President Trump vacationing in Palm Springs, the British Virgin Islands and French Polynesia; on Sunday, he is scheduled to appear in Boston to accept the Profile in Courage Award from the John F. Kennedy Library Foundation.