Since you are staying downtown, things to do downtown include:
1. Aquarium
2. World of Coca Cola
3. Tour of CNN
4. Centennial Park
5. Underground Atlanta (this really is not much any more - 30 minutes there usually does it!)
6. Tour the capital
7. Lots of restaurants. An interesting tourist experience is to eat in the revolving restaurant on the top floor of the Westin - it's the round high rise.

Take the train north to the North Ave Station:
1. Fox Theatre (try to get a tour - it is incredible)

Take the train another stop north
1. Tour the Federal Reserve (Monday - Friday only)

Walk (or take the train one more stop)
1. High Museum of Art
2. Museum of Design Atlanta (across street from High)
3. Take shuttle to Atlantic Station from Arts Center train station (Atlantic Station has shopping, dining, movie theatre, the Dialog in the Dark & Bodies exhibit)

Train also goes further north to Lenox, but that is just a regular mall - not what I would want to do on my vacation.

If you have a car and go north near Lenox, you can visit the Atlanta History Center and the Swan House.

From the 10th street station (near the Fed) you can walk east to Piedmont Park. Depending upon when you are in Atlanta, there are often events in the park. Otherwise, just a great place to walk. The Botanical Gardens are at the north end of the park.

About a mile east of the park (either alot of walking, and return downtown by cab, or if you have a car) is Virginia Highlands - lots of neighborhood restaurants & shops.

From downtown, you can walk just under a mile east to Ebenezer Baptist (both old and new) and the Martin Luther King Visitor Center. Tours of the birth home are well worth it.

From downtown, take the train to 5 points, then east to Decatur. There are lots of small shops and tons of restaurants in downtown Decatur. A great evening diversion. Downtown Decatur also often has festivals, etc.

If you do rent a car, about 40 minutes east of downtown is Stone Mountain Park. Climb the mountain if you are athletic (takes about 1 hour), or visit the amusement park or take a boat ride and stay for the laser show. Or you can walk AROUND the mountain. Costs about $8 to enter the park with a car. The amusements are priced either ala carte or a day ticket.

Also close to downtown but not on the train (there may be a bus) are the zoo and the cyclorama.

About 10 minutes from downtown via car or cab, also east, is Fernbank Museum of Natural History. It is about 1 mile from Emory University, also an interesting place if you have a car.

If you want interesting neighborhoods, Little 5 Points is east of downtown about 10 minutes by car, or if you want to walk, about 45 minutes from downtown on the very nice path (very safe in the day).

Also east of downtown near Little 5 Points if the Carter Presidential Center.

Your visit to many of these places will be enhanced by partaking the activities. So, for example, google the Carter Center to see if there are any events that interest you taking place there during your visit (check both the Carter Center site and the Carter Library site - they are next to each other but run separate programs).

Also check out the place that gives Segway tours of the city, check out the Atlanta Preservation Center walking tours (http://www.preserveatlanta.com/walking_tours_link) and google around to see if there are any neighborhood festivals during your visit.

Lots more to do, really depends upon your interests.