GO AHEAD and sign the guest book if you want to--it's given to the CIA every night," jokes the National Park Service guide as we file through the door that leads to the elevator that will take us to the top of the clock tower of the Old Post Office Building.

The glass elevator holds 15 people, and now that the tourists are thinning out the lines are short. The view from the tower is spectacular, but getting there is half the fun -- if a little scary. We take off and float upward through the vast open space of the Pavilion, watching the people sitting at the tables below us get smaller and smaller. The area below us, explains the guide, was once covered by a steel superstructure and used as a mail- sorting area. The steel structure screened the blue-collar mail sorters from the white-collar bureaucrats whose offices looked down on the inner courtyard. We look out at artist Robert Irwin's symmetrically hung white cotton squares. They are supposed to fill the soaring emptiness under the skylight by casting interesting shadows, explains the guide. They cost $80,000.

At the ninth floor, we stop, miraculously, and step out for a walk, through a bookshop, to the elevator that will take us to the tower. The oohs and aahs start on arrival. At 315 feet, the tower is not as high as the Washington Monument, but we are outside, though protected by screens, and the view is spectacular. From the west side, we look down on the red tile roofs of the Federal Triangle buildings and, beyond them, the Washington Monument, the Lincoln Memorial, the silvery skyscrapers of Rosslyn. From the north side, we see the Washington Cathedral and the National Shrine of the Immaculate Conception. From the east side of the tower, Pennsylvania Avenue leads the eye to the Capitol, the Supreme Court, the Library of Congress. From the south, the river gleams in the sunlight.

Just below us are the clock faces and the mechanism and the bells and, in order to see them close up, we walk down the stairs.

"The clock used to have a pendulum, but it fell off, crashed through three stories and landed near to somebody's desk," says the guide. "It was replaced with a motor."

The view of the clock faces from the inside is almost as exciting as the view from the tower, and just below the clock faces are the bells -- ten giant bronze bells made in Whitechapel, London, by the Ditchley Company, who made the Liberty Bell and the bells in Westminster Abbey. The bells are attached to ropes, which are pulled by bellringers from Washington Cathedral to celebrate national holidays and to mark the opening and closing sessions of Congress. The bellringers practice most Thursday evenings. There were no bells in the original Old Post Office, but the British gave them to us for the Bicentennial, and they needed a home.

As we descend in the glass elevator, we debate what we liked best -- the view to the outside or the view from inside of the clocks and bells. Nobody can decide, and there's at least one vote for the glass elevator.

SOARING TO THE TOP -- The glass elevator takes visitors to the top of the Old Post Office Tower every day between 9 and 5. It's free. OTHER KIDPLEASERS ON THE AVENUE

* Exhibition Hall at the Treasury Building, which contains exhibits on Fort Knox and carrying gold by stagecoach.

* Hot fudge Sundaes at the Old Ebbitt Grill.

* The ice skating rink in Pershing Plaza (scheduled to open as soon as it gets cold enough).

* The National Aquarium in the basement of the Commerce Department, especially at shark-feeding time (Mondays, Wednesdays and Saturdays at 2) or piranha-feeding time (Tuesdays, Thursdays and Sundays at 2).

* Only Happiness, a sticker store, and giant chocolate chip cookies from Hershey Cookies, both in The Shops at National Place.

* Have a Heart, for everything from mugs to T-shirts to pencils in the heart motif, and Circus Minimus, a toy store, both in the Pavilion at the Old Post Office.

* The FBI tour, complete with target practice. Weekdays only.

* The waterfalls in the cafeteria at the National Gallery of Art, with a chance to buy postcards in the adjacent shop.