In the cold drizzle yesterday, architect Hugh Newell Jacobson carefully shaped the top of the sand mountain into a tower.

"What kind of plans are you using?" asked the bystander. "It's an old architect's trick," said Jacobson. "Build first. Plans second."

Jacobson had another trick A day's consultation with the Bricklayer's Association yielded a substance for spraying the castle to protect it awhile from April showers. It's the same fixative used on airplane runways in sandy areas.

Jacobsen's ephemeral project in the garden of the American Institute of Architect's octagon house marked the opening of "Just for Fun" an exhibit of architectural toys that runs through June 17.

While Jacobsen's hands almost froze on his sculptor's spatulas, upstairs in the main gallery David O. Meeker Jr. had another problem. Meeker, AIA executive vice president, was close to asphixiation from glue fumes. Unlike Jacobsen, who was making things up as he went along, Meeker had an elaborate idea, or at least title, in mind. "An Evocative Office Structure for an International Communications Corporation"-a poke at Philip Johnson's AT&T building in New York City.

The idea was fine, but Meeker, like some other architects before him, and a hard time keeping the structure from falling down. Finally, he gave up and cheated by gluing the wooden columns and blocks together.

Washington architect Nicholas Papas took another poke at Johnson-this time at his all-glass-and-steel house. He "reinterpreted" it in Lincoln Logs (a toy, incidentally designed by John Wright, son of architect Frank Lloyd Wright).

The galleries were full of other architectural fantasies-scratch an architect and fined a mud mansion maker. A space colony made with erector sets by Architects Collaborative of Cambridge looked as though at any minute it might spin off on a Star Trek (from which the design was freely borrowed). Fishing Village, south of England, was constructed of modular blocks, the kind Frank Lloyd Wrighths mother brought for him, by Avery C. Faulkner with help from his son. And an Octagon made with Inkertoys, was put together by Donald B. Myer of the Fine Arts Commission.

The show also is full of antique toys, some from the collection of Jeanne Butler Hodges, president of the AIA Foundation, and her husband. Two mid-18th century doll houses are included.

Probably the most expensive toys were the huge North Church, made of more Lego blocks than the average family could afford in a lifetime, and the $1,000 Fischertechnics toy put together in a Frankenstein funhouse by Peter Ksietepolski, a partner of I.M. Pei.

And in one corner, the Cleveland Wrecking Co. toy waited. CAPTION: Picture, Hugh Newell Jacobsen, by Harry Naltchayan-The Washington Post