When President Reagan takes his historic ride down Pennsylvania Avenue from the Capitol to the White House next Monday, he will drive past Marriott's new flagship hotel -- booked a year in advance for this moment -- past plush new offices and the restored grandeur of the Old Post Office building.

He will ride on repaved streets, past sidewalks done in stylish brick and plazas adorned by 500 young willow oaks. He will see parks and $800 million worth of private development -- all additions since his first inaugural ride four years ago.

"Next Monday we will see a much prouder avenue than has been there for 100 years," said architect David Childs, who came to Washington in 1968 to work for the men who started the massive improvement effort. "And after the celebration is over and the parade-watchers go home, it will still be an avenue that has shops and stores, restaurants and people -- a happy, animated place to be."

And if there is a night-and-day difference in the boulevard itself, so, too, is there a difference in the people who will watch the parade from scores of new windows.

"We've got the people who make things happen -- bedrock Republicans," said one employe at the J. W. Marriott Hotel that overlooks the avenue at 14th Street. It is those customers who can afford the tab that starts at $135 a night and who began booking rooms when the hotel was a hole in the ground, she said.

Gone are the buildings down the avenue that housed hole-in-the-wall restaurants such as Barney's Delicatessen, where in years past waitresses turned away from frying hamburgers to watch each new president pass by.

In their place are the lawyers and lobbyists, the architects and public relations people who occupy the new buildings. At 1201 Pennsylvania Ave., with its soaring 12-story atrium and its lobby lush with palms and ferns, the tenants include Covington & Burling, the city's largest law firm; Skidmore, Owings & Merrill, where architect Childs is a partner, and the public relations firm of Hill & Knowlton.

In the building there also is a fitness club, a restaurant offering a hamburger and french fries for $5.75, and a French bakery that sells croissants and kiwi fruit tarts.

But some things on the avenue, such as politics, never change.

The District's Department of Employment Services, which has occupied the corner building at Sixth Street since 1961, will be closed on Inauguration Day, and its workers have no regrets about staying home. "I'm just delighted we're going to get a holiday," said Madelyn Andrews.

More than 85 percent of D.C. voters supported Walter F. Mondale in the November election, while all but one of the 50 states -- Mondale's home state of Minnesota -- were won by Reagan.

"Maybe if the Democrats had won, there might be a lot more people who wanted to be here," another employment services worker remarked. "But who wants to come to cheer for them?"

Many across the way on the south side of Pennsylvania Avenue do -- those in the massive buildings of the Federal Triangle, built in the 1930s. The heads of many federal agencies plan to hold parties for top officials and guests. And some of the rank and file will be there.

Mark Horschak, a Federal Trade Commission attorney, said he will be watching from his office window. "This will be my first Inauguration Day here," he said, "and I want to see it all."

But at the National Archives, between Seventh and Ninth streets, a drawing for Inauguration Day passes was called off when only 165 workers from the staff of 500 applied for the 225 passes.

Walter Riggin, proprietor of the B & B Newsstand at 13th Street, is looking forward to Inauguration Day. It will bring "delicious" tourists, he says, to browse through his cellophane-wrapped pornographic magazines and his $2 buttons that proclaim "I've Been for Reagan" and "Blondes Prefer Reagan."

Riggin's stand is the only one that remains of perhaps a dozen that offered such wares four years ago. "Nobody around here will accept us anymore," Riggin lamented. "When you say it's a newsstand with video games, they all get uptight. They don't want the environment. But you know, we get your lawyers and administrators and your everyday professionals. That's who mostly comes in here."

Riggin said he has been ordered to vacate in mid-March along with the other tenants, as the building undergoes a total renovation, part of the next phase of the boulevard's revitalization. The avenue still is teeming with construction and rife with open sites and half-done projects -- all under the tutelage of the federal Pennsylvania Avenue Development Corp.

When President Kennedy rode along the avenue in his 1961 inaugural parade, he looked out at the boarded-up shops and dreary buildings and decided that something had to be done with the "Avenue of Presidents," as the sweeping boulevard was dubbed in better days. Although it is not yet the "national Main Street" Kennedy envisioned, it has come a long way.

And, as always, it reflects not only the accomplishments but also the problems of the federal city.

When federal offices and private buildings along the avenue began planning for Inauguration Day, they were visited by Secret Service agents who warned that no one must open a window or go out on a roof. "It was clear," said one official, "they were very worried about security and about terrorism."

Lafayette Square, across from the White House and traditional end point for the inaugural parade, has changed from an oasis of fountains and flowers to a site for permanent protest and a gathering place for Washington's homeless. Street people are fed there every evening from a church-sponsored truck, and a few weeks ago a homeless man froze to death there as he slept on the ground.

Reagan will watch the parade from an enclosed reviewing stand across from the square, and the park will be the site of bleacher seats that were sold to inaugural guests for $75 and $100 apiece.