Don't leave the downtown area without going into the Atlanta Fulton County Library at 1 Margaret Mitchell Square to take a quick look at the exhibit of personal items that belonged to Margaret Mitchell and the typewriter on which she wrote ''Gone With the Wind.'' Other exhibits at the library this sum-mer include a showing of Charles Sebree's original wash drawings for Countee Cullen's book, ''The Lost Zoo'' (to Sept. 6) and Haitian art and oil-drum sculptures (to July 30). The library is open Monday and Friday from 9 A.M. to 6 P.M., Tuesday, Wednesday and Thursday from 9 to 8, Saturday 10 to 6 and Sunday 2 to 6.

For lunch, all of the downtown hotels from the Hyatt and the Marriott to the Hilton and the Westin offer a variety of restaurants. Notable among them are The Cafe and The Restaurant at the Ritz-Carlton (181 Peachtree), where lunch runs from $10 to $25 downstairs at The Cafe and $23 upstairs at The Restaurant, and the Cafe de la Paix with its sumptuous buffet at the Atlanta Hilton, where lunch or Sunday brunch is $10 to $25 a person. But for a sampling of what downtown office workers do for lunch go to either the Foodworks Food Court at the Georgia-Pacific Building or the Food Court on the lower level at Peachtree Center, an office complex just a block or so north on Peachtree Street and Harris Street. These indoor galleries with their cornucopia of stalls offer inexpensive lunches and relative relaxation amid the lunchtime bustle. The range of food runs from pizza and Chinese dishes to quiche and Southern suppers of chicken, yams and greens. Midtown

The closest of the neighborhoods to downtown is the midtown area to the north, an older residential area with a host of renovation projects and new construction that has made it a thriving commercial center as well the focal point of the performing and visual arts. While the area includes Atlanta's older, Moorish-style Fox Theater, the real catalyst for its emergence is the Robert W. Woodruff Arts Center, the complex that houses the High Museum, the Atlanta Symphony Orchestra, the Atlanta College of Art and the Alliance Theater. Nearby is the Academy Theater (173 14th Street N.E.; 404-892-0880).

The High Museum of Art (1280 Peachtree Street N.E.; 404-892-4444) is the city's much-praised modernist building designed by Richard Meier. The building houses a good general collection and will add some special exhibits during the time of the Democratic Convention.

One exhibit features sculptures, photographs and paintings of the Harlem Renaissance; it runs through Sept. 4. Also running until Sept. 4 as part of the museum's ''Art on the Edge'' series featuring works of artists who challenge traditional concepts, is an exhibit by Sherrie Levine. The museum is open from 10 A.M. to 5 P.M. every day except Wednesday, when it stays open until 9 P.M., and Sunday, when it opens at noon. Admission is $3; children under 12 are free.

It's a nice area for casual dining as well. Try the small and intimate Taste of New Orleans (889 West Peachtree Street N.W.; 404-874-5535) with its creole and Cajun specialties, most notably the saucy red beans and rice with andouille sausage. Or head over to the Sierra Grill (1529 Piedmont Road N.E.; 404-873-5630) for southwestern specialties in a spacious room. Reservations are necessary for both and both have entrees ranging from $10 to $25. Neighborhoods

Beyond midtown is Buckhead, an upscale area where the large homes of some of Atlanta's wealthiest citizens sit on spacious, landscaped lots. It is also the home of the city's first and still premier shopping mall, Lenox Square.