REBUILDING and enlarging the sidewalks of Pennsylvania Avenue was no problem; the real challenge nowadays is keeping them from folding up at night.

At least one address on Pennsylvania Avenue -- aside from the famous banquet hall at 1600 -- is doing its part.

Over at 1201 Pennsylvania Ave. NW, they now answer the phone with a cheerful mouthful: "Good evening, Chez Artiste, formerly known as the Bread Oven." But this French restaurant-supper club, with its clear-Lucite Kawai grand piano in the front window, has more than a name change to be cheerful about: Last week it replaced an aging, amorphous cabaret show with a sprightly, energetic and thoroughly entertaining new Thursday-Saturday revue of American popular song.

Titled Cabaret Americain (and alternating Saturday nights with the musical political satire of The Capitol Steps), the show employs a facile, fast-moving cast of six, including director Carole Lehan, whose quest for musical authenticity pays off in handsome (and generally three-part) harmony. There's also some particularly strong and memorably spoofy work by singers Allison Briner and Gary Best, and Patrick Wiley's smart and streamlined costumes contribute to the sophisticated overall feel of the revue, which covers music that spans the radio theater of the '30s to the sit-ins of the '60s.

The cabaret starts at 8:30 Thursdays, Fridays and alternate Saturdays. (This Saturday it's the Steps' turn; pianist-humorist Dan Ruskin still pilots the Kawai Mondays through Wednesdays.) For dinner reservations (entrees are $14 to $18; a $5 cover otherwise), call 737-7772. BABES IN MUSIC LAND

We were just going to mention that Babes, the many-lived tavern up at 4226 Wisconsin Ave. NW, has recently rediscovered live music -- what with this weekend's two-night reunion of Harry Traynham, Jack Bond and Brian Goddard -- when, suddenly, the phone rang. And it was Michael Mann, former booking agent of the former new-music venue/Chinese restaurant Ted Liu's, who was calling to say he would soon be booking bands onto the stage -- long ago converted into a railed dining platform -- every Friday through Sunday night at Babes, of all places. (And actually, Babes almost has been all places, having spent parts of its 10-year life as a rock showcase, a disco, a piano bar, a ribs restaurant, a nice outdoor cafe -- which it still is -- and a broadcasters' hangout.)

"We're going to give it a shot," said co-owner Glen Leizear, who acquired the 140-seat Babes several months ago with partner Earl Klein.

Mann's shows -- which will bear an odd resemblance to those he'd already booked when Liu's closed April 2 -- tentatively start Friday, April 25 with B-Time and Little Big.