Tourists who leave Philadelphia with memories of aged documents and historic builkings have not seen the real city. The more I think about it, anyone who leaves without at least two over-laden shopping bags from the Italian Market, the lingering taste of a fish cake smashed onto an Old Original Levis' hot dog and images of a dozen Art Deco storefronts has missed the city's essence.

What follows is an essential guide to the Philadelphia that most visitors, foot-weary from the expanse of historic sites, rarely encounter. This Philadelphia begins at South Street, the city's main shopping district in the l930s, never leaves the ethnic neighborhoods and politely ignores the newly renovated Bourse and other well-publicized treasures of Center City.

Delights like the aged crooners at the Triangle Tavern, the mussels at the South Philly Grille and the interior of Jim's Cheese Steaks, should not go unexplored simply because no brochures or travel agents point you in their direction.

I first learned of the joys of Philadelphia's commercial life three years ago through Lenny Davidson, a neon artist and native son who was recruited to lead city planners, architects and preservationists on a restaurant and neon adventure. His advice to would-be explorers included hints such as "ties are suspect in South Philadelphia. Remove sterile words like 'process' and 'sociotechnical' from your vocabulary."

Davidson, who still offers tours when time permits, treads gently when mentioning his favorite haunts. "There are thousands of people in Philadelphia who could get into fights about where to get the best hoagie or cheese steak or pizza in town," he said. "These are very passionate issues in Philly."

Because Davidson's grand tour includes 85 stops, from the Goldman Theatre marquee at l5th and Chestnut, to the best ribs in town at Lester and Bea Bea's Lawnside BBQ at South Street between l6th and l7th, I devised a scaled-down two-day version that can be done on foot. (Although I jogged, complaining for at least 20 blocks, to breakfast at the all-chrome Melrose Diner at 15th and Snyder, exercise, my companion agreed, was futile after the custard-baked French toast and scrapple arrived.)

We began our weekend escape by taking the slow train to Philadelphia. Although I normally enjoy the two-hour ride, happenstance put us on the 8 p.m. train on Friday night, the first train of the cheap, $35 roundtrip fare.

The train was hot, crowded and we had to ask a kind soul to switch seats so we could sit together and split our picnic dinner. But we got engrossed in a young man's comic attempts to impress a seatmate and amused by an effervescent toddler, and soon we were at 30th Street Station. We took a cab to the Bellevue Stratford Hotel, which I chose out of curiosity to see how it weathered the outbreak of Legionnaire's Disease, which closed the historic building from July, 1976 to 1978.

The hotel is now owned by the Westin chain and is in spectacular form; plush, but tasteful. The special rates for Friday, Saturday and Sunday nights are welcome: $59 a night for two, with children under 18 free. Our large, quiet room was decorated in a striking federal yellow wallcovering, and included a writing desk and an armoire that hid the television set, an appealing touch. Nothing South Philly about it, but it is only a 10-minute walk to South Street and is on bus lines and near the subway.

Morning brought a walk to the Famous Deli for breakfast, at 4th and Bainbridge streets. Established in l923, the third generation of Auspitz brothers still run the restaurant. Famous is heavy on black and white floor tile, porcelain freezers and corned beef, blintzes and chopped herring sandwiches. We opted for bagels and scrambled eggs.

In l963, South Street was immortalized in song by the Orlons, as in "Meet Me on South Street." And meet people we did, from the window shoppers in the city's diamond and wedding apparel markets, to serious purchasers of art at The Works, an art gallery at 3l9 South St. with everything from New Wave electronic jewelry to fine glass sculptures.

South Street "musts" include Rocketships and Accessories, at 623 South St., a unique store that features the world's largest collection of ray guns, Captain Video helmets and toy robots. Owned and designed by architect Joel Spivak, 44, (yes, he used to get talk show host Joel Spivak's mail when both were in Philadelphia), the clerks wear gray and red space suits and work under an arch composed of radiators and garbage can lids. The shop is filled with the urgent whine of ray guns being tested by happy children. "It's like music," Spivak says tolerantly.

Across the street, at 6l2 South, is Capricorn, your indespensible one-stop shopping for pillbox hats, tulle prom dresses, pastel fur muffs and all other accoutrements of the fast lane in l950. Proprietor Bill Novak has assembled-- well, stuffed--some of the least expensive finds of the era into several display groupings. Rhinestone jewelry fills an entire corner, right next to the lame' jackets.

Sticker fanatics, as most boys and girls under ll appear to be, will appreciate Paper Moon, at 329 South St., and its never-ending rows of shimmering, Day-Glo stickers.

Also in the neighborhood, on the Fourth Street fabric district, is Lily's Silks, 7l6 S. 4th St., a store with women's designer clothes that are far cheaper than Loehmann's. Across the street is Night Dressing, for lingerie.

When lunchtime rolled around, we met friends at Jim's Steaks, the 4th and South institution whose frying onions advertise themselves for at least a block in either direction. Jim's black and chrome Deco decor is secondary to the main attraction: the sandwich makers on the grill line who haven't missed a beat since the doors opened in l939. As you order, be prepared to shout "with peppers" or "extra onions" and don't be appalled that the cheese is Cheez-Whiz. That distinguishes a genuine Philly cheesesteak, as much as does the spectacular submarine-style bread, made by Amoroso's, a local bakery. At $2.20, this is a lunch.

You won't still be hungry, but walk the two blocks to Levis' at 6th and Lombard to see the nation's oldest soda machine dispense champagne cherry ale (50 cents). The hotdog neon sign emphasizes Levis' slogan, "World's Best Hotdogs Since l895." Look at the wall plaques to the dozens of the people in the restaurant's "50 Year Club," stomachs that have known a half-century of loyalty to one of America's oddest combinations: a hot dog with a fish cake. (

.4l.)

Because it only seemed proper to honor the man whose experiments with electricity resulted in a city full of neon, we took a half-hour walk to Benjamin Franklin Square, at 6th and Vine streets. Don't watch the silly National Park Service movie about Franklin; it is done in the style of elementary school pageants. But the basement exhibits do detail the unconventional and inspiring life of this tireless patriot. And, in an imaginative marriage of avant-garde art and history, one mirrored room is devoted to neon signs that flash colored words representing Franklin's varied talents: inventor; citizen; diplomat; publisher; etc.

We made a slight detour to the wharf area to Shane's Candy Shop, at ll0 Market St., but only because our friend's child insisted. Since l9ll, Shane's has been selling special confections like cinnamon potatoes and oversized marshmallows from its mahogany and leaded glass counters, all within sight of the blue Ben Franklin Bridge over the Delaware River.

Exhausted after a day of walking, we rested for two hours before tackling the rigors of dinner and neon in its proper lighting. Because the night was chilly and we were tired, we took a cab to a neighborhood family restaurant and bar, the South Philly Grill, at l2th Street, between Snyder and McKean. The jukebox here used to play only Frank Sinatra, but a few others have crowded in, although Frank's pictures (oil painting on black velvet) line the walls. The mussels are the big attraction here, fresh, soft and bathed in a garlic broth. An order at $3.95 is a perfect appetizer for two.

We thought we'd walk off the mussels by the time we hit beautiful tile-facaded Marra's, the city's oldest Italian restaurant, at l734 E. Passyunk St. We were wrong, and the small pizza ($4.25) we ordered could have fed six. Like all of the South Philly restaurants we visited, Marra's has a well-deserved reputation for neighborhood atmosphere, great food and cheap prices.

Next was the Triangle Tavern, at l0th and Reed. Some might call this place seedy. Lenny Davidson calls it a living museum of South Philadelphia. "If a sociologist awoke to find himself here, he could easily believe he was in heaven," Davidson said. Although we didn't possess enough energy to stay for the show, most everyone else in the immediate world did, to gape at the middle-aged Rod Stewarts singing off-key with a polka band. There's a lot of noise, a lot of drinking and it can be an unforgettable night if you're with a group of friends.

Leaving the garish bright lights, we took a cab home and felt like interlopers padding down the thick hallway carpets.

After breakfast at the Melrose, l50l Snyder St., (with its l950s murals of Philadelphia), we walked the nine blocks to the Italian Market, at 9th and Catherine streets. The market is open 8 a.m. to about 4:30 p.m. Saturday and from 8 a.m. to l p.m. on Sunday. As a huge cloth shopping bag became crucial early on, remember to bring one from home. The fresh pasta, frozen ravioli, etc., must be refrigerated within six hours, so plan to make those purchases right before leaving. Or, as many friends who drive up do, take a cooler with ice.

The excitement, smell and taste of the Italian Market is so appealing that I have been known to take the train to Philadelphia just to load up at the market and return, arms near the breaking point, in the evening. (It also makes me less homesick for the values and quality of my native Cleveland's ethnic West Side Market.)

Tops on my list at the Italian Market is cheese ravioli from Superior Ravioli, 909 Christian St., fresh figs from the Spice Corner, 904 S. 9th St. and the world's best cannoli from Litto's, at 9th and Christian. (Try your own test with the other top cannoli contenders at Isgrows, l0th and Christian, and Termini at l523 S. 8th St.)

You can also compare the artistry in various neon cows. Find them at the Reliable Meat Market, 9th and Federal; Leuro Meats, l0l9 Meats and Frankie's Choice Meats, all on 9th, between Washington and Carpenter. There's no cow, but everything else is in neon at Esposito's Meats, also at 9th and Carpenter.

True to form, with arms loaded, we trudged back to the hotel, took a cab to the train station and sped home to Washington, with memories of much more than Penn's Landing and the Liberty Bell.