PHILADELPHIA IS GIVING herself a 300th birthday party this year, right smack in the middle of a recession. Is anybody going to come?

Philly's sure they will, says Paul Decker, vice president for tourism and marketing of the Convention and Visitors Bureau, because this party isn't just dry history; it's activity oriented.

"You can't just give a 300th birthday," says Decker. "Who cares? Not the guy from north Jersey. But he cares if it means an opportunity to tour the Tall Ships."

If activities will sell the party, it's already an assured success. The QE2 will be visiting April 25-28, welcomed by bands and costumed Mummers. May 1 and 2, the "Taste of History" tours of Fairmount Park will showcase the city's restaurant renaissance, and on the 7th through 9th the Folk Fair will bring ethnic foods, entertainment and crafts to the Civic Center. On May 23, one hundred of the city's finest restaurants will serve their specialties in a Restaurant Festival on the Benjamin Franklin Parkway in the shadow of the Museum of Art.

June 17 the Tall Ships--some 40 of them in all--will sail up the Delaware River to a reception at Penn's Landing, including tours of the ships, a parade and fireworks. They'll be open June 18-20 from 10 a.m. to 6 p.m. From June 30 to July 11, strolling musicians will give daily concerts of American music from classic to rock and, on the first three weekends in August, the city is planning super-style block parties in five different neighborhoods each day with plays, music, historic tours and a smorgasbord of ethnic foods.

Late in October, "William Penn" himself will arrive on a recreation of his barge to dock at Penn's Landing. The whole program bids fair to make his image atop the old City Hall bust its waistcoat buttons in pride.

Is this going to mean prosperity for Philadelpia and the end of W. C. Fields-inspired jokes about its image? Is tourism going to boom? Tom Lynch, senior vice president of Philadelphia Industrial Valley Banks, thinks it will.

"You know the old theory that money brought into town is spent five or six times over, from the cab driver on down," Lynch says. "Things are tight, but we're hopeful. People may have to reduce their plans or lower their horizons, but I don't think they like to stay home."

The primary market of Century IV is within 300 miles of the city, which means that most of the visitors can just get in their car and come. Philadelphia hotel rates have been depressed and are now rising only moderately compared to other large cities. Finally, the bus tours, accustomed to flying visits limited to the Liberty Bell and Independence Hall, are interested in the special events planned and may find themselves hooked on Philly when the party's over.

Pack a couple of pairs of walking shoes--this is a town to see on foot--and give Philadelphia the once-over before the crowds arrive.

Philly is our fourth largest city, but you don't feel blown away by its size because the streets are mostly narrow and pretty little parks are hidden away in unexpected places. It reminds you of London so it doesn't surprise you at all to see horse-drawn conveyances clopping along. By unwritten law, nothing can be higher than William Penn's head atop the Versailles-inspired City Hall, but since that's pretty high you have urban canyons down which the wind from two rivers sweeps hard enough to swirl up the trash in the streets. The subway system is dingy and dark, yet the streets are safer than in most cities of its size, they say.

The Bellevue Stratford, the 76-year-old hotel where Legionnaire's disease was born, is now completely renovated and has even taken back her old name. She looks splendid. The beautiful old ceilings have been lovingly restored, chandeliers sparkle, and afternoon tea in the lobby is becoming a Philadelphia custom. All this is especially heartening since, back in 1976, the doors were chained shut and the last guest (a gentleman in a raccoon coat and derby) drove the head elevator starter home in a 1920 Cadillac, marking the end of an era.

The Warwick is Philadelphia's deluxe modern hotel, featuring chrome and blue plush furniture in the lobby and a branch of the disco, e'lan, adjacent to the reservation desk. You can sit in the Warwick lobby and watch space-age chic parading through en route to e'lan, but give me instead the wonderful Barclay Hotel, small, elegant and restrained. The Barclay has some permanent residents, as well as transients, and numbers among the former the city's beloved Eugene Ormandy.

"Meet me at the iggle," Philadelphians say when they want to set up a rendezvous in the center city. They're talking about the atrium of John Wanamaker's store where the huge statue of a fierce eagle crouches under the pipe organ which daily plays concerts for the noonday drop-in trade. Wanamaker's is no Bloomies but it's as much a Philadelphia tradition as the Liberty Bell.

If you haven't seen the famed bell, or Carpenters' Hall where the first Continental Congress met, and the rest of the historic landmarks of our nascent government, you can look them over at Independence National Historic Park where they are conveniently grouped for a star-spangled square mile. It's quite a walk through history. Halfway through my tour I had to go back to the city-owned garage across from City Tavern where I had parked the car and get a second, more comfortable pair of shoes.

Independence Hall is currently dressed in scaffolding for inspection and repair, but the 20-minute tours inside still go. The Liberty Bell is now housed in a small glassed-in enclosure where it is easier to see--you can lean right over and feel the crack. You'll also see the First Bank of the United States, and the second, and the Merchant's Exchange, but the attraction that captures the imagination is Franklin Court.

Not realizing the importance of Benjamin Franklin for the ages, his heirs tore his house down, so the city of Philadelphia was reduced to recreating it as best it could in a skeleton outline. On the floor of this house without walls are engraved quotes from letters Franklin wrote his wife Deborah during its building. "Have you moved everything," he inquires anxiously from Paris where he was on a diplomatic mission, "and put all papers and books in my room, and do you keep it locked?" Stand there reading that worried letter and all the Franklin statues somehow come to life.

Don't fail to descend the ramp in the adjoining building to see the other Franklin exhibits. Some of his furnishings are here: a Franklin stove, a four-sided music stand he invented for a string quartet, a library chair with foldaway steps. After this comes a strange room composed of mirrors and flashing colored lights spelling out Franklin's various roles--statesman, diplomat, inventor, you name it. Persevere through this disco-looking chamber to the wonderful room beyond, where you can pick one of a bank of phones and "talk" to any of Franklin's admirers whose telephone numbers are conveniently listed on the wall (including the proper area code).

I chose Honore' de Balzac's name from the wall and dialed the Paris number. He answered immediately but his French accent confused me so I hung up and called Herman Melville in New York, who was a bit long-winded about his friend's accomplishments. There was so much praise, I longed for a hint of something a bit derogatory--a terrible temper, perhaps, or a phobia about cats. But it was not to be.

The Bourse, the city's new mall of high fashion, is handy here if you need a rest from history and there's a branch of the Roman Alfredo's in it; if you want to remain in the period you probably should lunch at the City Tavern, which John Adams called "the most genteel tavern in America." It operates now as a 18th-century tavern with costumed waitresses and moderately-priced food. It's no five-star restaurant, but when you're dipping into your crab cakes remember that it was here, on May 20, 1774, that Paul Revere came with news from New England that Parliament had closed down the port of Boston.

The five-star restaurant of Philadelphia in the Mobil Travel Guide is Le Bec Fin on Spruce Street, and you must have reservations way ahead. The Frog is one of the well-thought-of newcomers. For a bargain good meal, try Upstairs at the Commissary. The Fish Market at 18th and Sansom streets. is justly renowned. At Bookbinder's, ask for snapper turtle soup, a Philadelphia specialty. For a snack, try the other indigenous treat, soft pretzels, available everywhere in vendor wagons.

Society Hill is the Georgetown of Philadelphia, with cobblestone streets, brick sidewalks and handsome Franklin street lights. It is being extensively restored, but another Philadelphia landmark, the Lit Brothers' striking but decaying Victorian-era department store on Market Street, has a more uncertain future. It's worth a few minutes of your time to drive by this rundown beauty which has been empty now for nearly six years and may not always be there. Among the museums are the Rodin, the Atwater Kent Museum with early city history exhibits, and the Norman Rockwell Museum in (where else?) the old Curtis Publication building.

One endearing fact of the city is the law which orders that 1 percent of the cost of any new building must be donated for public art. The results are everywhere, even in traffic islands, and they are of all stripes. Second only to the Wanamaker eagle in the affections of Philadelphia is the Claes Oldenburg steel "Clothespin" that rises at the corner of Market and 15th streets.

During Century IV, Philadelphia hotels are offering special package rates. Contact the Philadelphia Convention and Visitors Bureau, 1525 John F. Kennedy Blvd. 19102, or call toll free 1-800-241-8444. Last year the city had 3.5 million visitors who arrived by auto, bus, rail or air, and this year will certainly welcome more. Better call ahead.