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The nearest MARTA stop to the hotel is Peachtree Center. A few stops north and you'll be in Midtown. If you go one stop South (Five Points station) you'll be able to switch to a train running eastward to Decatur, 6 stops away.
If you're really committed to seeing the best Atlanta has to offer, however, it's probably fair to say that a step up in the transportation budget is called for. A $15-$20 cab ride can get you out to West or NW Atlanta, which has become a haven for foodies and mixologists in recent years, as well as a preferred spot for boutique shops, galleries, etc. Or you could just make friends with a local and bribe them into service.
Atlanta's downtown and midtown districts also have good Zipcar coverage, if you have a membership.
Downtown
How to get here: if you're at the conference hotel, you're already there!
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