Nancy Reagan, looking like a Christmas ornament herself in a red suit and red blouse, showed off her old-fashioned Christmas decorations yesterday at the White House.

She said the president has asked for such Christmas gifts as a jeep and a tractor, but that she didn't promise to have Santa bring them. Mrs. Reagan added that when people have been married as long as she and the president have been, they tend to give each other practical gifts. "I got a saw one Christmas," she recalled, "and we painted the kitchen another."

Mrs. Reagan said she hung ornaments -- beginning with a round ball inscribed "California" from a Nixon Christmas tree -- for two days, "but not all day, of course." But she did not put the Gabriel ornament on top of the tree. "It's too tall," she said, shaking her head at the thought of climbing to the top of the 19 1/2-foot tree.

The Reagans will decorate the tree in their private quarters on the second floor of the White House next week. Nancy Reagan expects the president to help -- he's pretty good at it, she says. The tree won't have the Reagans' own ornaments, though. "By mistake all our ornaments were sent to storage. I'd have to go through everything to find them, and I'm not in California. It's a shame, because I have all our decorations, even those paper ones the children made in school."

For the first time anyone could remember, the first lady offered the 75 or so members of the press eggnog, cookies and small sandwiches after the press tour. In conversation, she said someone suggested she don a disguise to go Christmas shopping, the way Pat Nixon and Jacqueline Kennedy did. "I haven't yet," she said, "but I did sneak out to Tex Charles B. Thornton's funeral at Arlington, and no one knew." Thornton, who died Nov. 24, was chairman of Litton Industries and a Medal of Freedom winner.

Mrs. Reagan showed off the Blue Room tree ornaments, made by craftspeople organized by the Museum of American Folk Art in New York; the doll house of interior designer Aline Koplin Gray of Philadelphia; the cre che given the White House in 1967 by Mrs. Charles W. Engelhard Jr.; and the gingerbread house, with a new jellybean walk, by White House assistant executive chef Hans Raffert.

Besides the tree in the Blue Room (emptied of its furniture for the occassion), trees in the great cross hall were decorated with real candles, and six more in the East Room were ornamented with white lights. The mantelpieces have greens, lotus pods and apples. Trees made of apples atop silver ice buckets ornamented one serving table. In the State Dining Room, the gold Monroe plateau serves as the Christmas centerpiece with holly and greens in the vermeil urns. The decorations are the work of Dorothy Temple, the White House florist, and 33 volunteers.

The Reagans plan to stay in the White House over Christmas, she said, with most of their family with them. Some California friends who used to entertain the Reagans at Christmas will join them, too.