"Oh, we had some great plans for the Pension Building. We wanted to turn it into a museum of play. All that space.We were going to have a helium balloon that you could ride, and a ferris wheel and a merry-go-round, and the attendants would have special skills, like juggling . . ."

Benjamin Lawless starts jumping around in his chair when you get him talking about his museum for play.

And it is safe to say that not a single one of the 2,750,000 people who pressed through the Smithsonian's Museum of History and Technology last summer and the nearly 3 million expected this summer gets more excited about those exhibits than he does.

In his wonderfully cluttered office sprawls a mannequin wearing a World War II uniform with the mud still on it. And a model of the Kon Tiki. And an Indian leather mask, an airship made from a toilet float, a Saul Steinburg drawing, a sand painting, tops, boat models, plane models, horse sculptures, seashells, games, all kinds of things.

"We certainly study work," he muses. "Why not play? We did the Sears toy collection, and that was okay, but the subject itself eludes us. Scholars are leery of studying play."

Present plans call for the Pension Building to become a museum of architecture.

Lawless is so high on play he once made a film, "Toccata for Toy Contract," addressed to the noted designer Charles Eames in an attempt to get him to exhibit his toy collection at the Smithsonian. Eames, in his 70s, is even more passionate on the subject than Lawless, and if you take him on at marbles, you'd better not be just fooling around.

Well, you win some and you lose some in the exhibits business. At the moment Lawless is working up a display on both world wars, from the GI's point of view: the K rations, the canteens, the unbiquitous khaki (even the foot powder cans were khaki-colored). He admits he'd had to drag the uniforms behind a car to give them the proper lived-in look. But aside from that, everything will be very, very authentic, for he has tough critics.

"You should see a family bunched in front of a case with some K rations on show. There's Dad. Maybe a little bald by now, pointing out all the items, and if you've got one wrong, watch out."

It is this ability to connect with the audience, to touch some common experience, and not merely to arrange things prettily, that makes a good museum. One vastly successful exhibit was the recreation of the 1876 Centennial show in Philadelphia, done meticulously in period style, from its dazzling cutlery display, laid out as a 19th-century merchant might have done it, to a precursor of the gasoline engine accompanied by its original innocent label describing it as an efficient new type of pump.

"We had 25 curators working on that thing," says Lawless. "We started on it a long, long time ago, and you wouldn't believe the numbers of people who didn't want us to do it. Also the people who would die for it."

The show, which covers 60,000 square feet, has been called the most satisfying exhibition ever mounted by the museum.

"Oh, the terrible fights we had over that. We screamed, threatened, died, cajoled, worked nights - it could be scary in there, you kept feeling you might meet some old Victorian gentleman in the dark. We knew it could be the most perfect exhibit ever, if we did it right. And people came out of the woodwork to help. Museums all over America contributed to it."

The Centennial show was, he adds, "the most painfula and delightful experience I ever had. It was a trip in time."

An award-winning filmmaker. Lawless put together a promotion film for the show, using the talents of some Arena Stage actors.

The art of designing exhibitions has come a long way since the days when dusty objects were set out any old way to languish on shelves in dim rooms. One of the first efforts to reach out to the public was Samuel Langley's children's museum, established in 1900. Langley, the institution's third director, built a charming room full of bird cages and aquariums and light.

And when he spotted a label for the European robin, 'Erithacus Rubecula (Linn),' he tore it away without a word and right then and there wrote his own label: "Robin Redbreast, one of the best beloved birds of England."

Every new move met resistance. The idea of showing airplanes by actually suspending them in the air was so disturbing to some curators that they refused to walk through the room "underneath those danged flying machines."

In fact, it wasn't until after World War II, according to Lawless, that the Smithsonian really took off, with the help of heads-up young staffers: instead of brass plaques, they now used big silk-screened posters; spaces were shifted around constantly; touchable exhibits were encouraged; the people were involved in the shows, not only with audiovisual fetures but even artificial odors, such as the smell of chocolate in a 19th-century sweet shoppe, barnyard aromas for a farm show.

"We still haven't learned to do it all right," Lawless said. "The period rooms are okay, but some of our big exhibits aren't as fascinating as they could be. Those huge old farm machines - you've got to get across the notion of how great they were to the people who had them, what tremendous amounts of work they saved despite the problems they brought. How do you express that idea?"

In two or three years he hopes to develop a hall at History and Technology dramatizing the transition from muscle power to machine power, and what it has meant.

A machine doesn't tell its own story the way, say, a sailing ship does, just by being there. You look at a Stanley Steamer and you say, uh huh, but you have to ride in one to appreciate the eerie quiet of its motor and its astonishing powers of acceleration.

There was the man who built a car, soon after the earliest models came out, that accelerated so fast it tended to fly. There was the guy who used a souped-up 85-mhp locomotive to chase thunderstorms in: The Smithsonian has the locomotive, but how do you demonstrate what's so special about it, aside from a placard?

"When Ford built his first car engine, he ran it once. Just once. Said, okay, so it works. Then put it away. We've got that engine downstairs." The man who makes a study of play shakes his head in bafflement. "Wouldn't you think he'd want to mess with it? Play with it? See what it could do?"

Lawless started out to be an engineer. When he went off to World War II his father, a veteran of the first war, gave him a sketch pad "so I wouldn't waste my time with women and cards" and when he came out he switched to art school, did his graduate work at the University of Illinois, tried his hands at directing a museum in Saginaw, Mich., and more or less fell into the Smithsonian during a visit here in 1953.

"My first thought was: 'Does this place need help?' I said I'd just work here six months, redoing the First Ladies' Hall. We spent our time cutting away lace that had been sewn over the plunging fronts on those gowns. Then they learned I could build models, so I kind of got involved."

He's been there 26 years.

He still sketches constantly. Once he went to Cape Canaveral for a rocket shot, and when everybody else had taken their photos and gone away he stayed behind with his sketch pad. The MPs arrested him five times before someone fitted him out with a huge badge that assured them he was one of ours.

"Museums get you into those things," he says. "You've got to be interested in what's going on in the world.Our young curators are in the field all the time collecting everyday Americana. And it's hard, it's not like some rich guy collecting things. Our people have to look at some rusted old engine and decide whether it's worth saving. You have to make those decisions. Muhammad Ali's gloves, Ken Kesey's acid trip bus: He's offered it to us, but we're still thinking."

Like other Smithsonian staffers, Ben Lawless hates to hear that old line about "the attic of the nation."

"My job," he says, "is to bring us out of the attic."