There is a Marta park and ride lot near Stone Mountain, I have parked there to ride into downtown. Marta will get you close to destinations, but you will still have to walk several blocks in most cases. I highly recommend using Marta as parking downtown is expensive and not necessarily closer than Marta stations. Look up the maps online before you go and plan your stops accordingly. Do remember that you are in a city and so should take safety precautions accordingly. Some will say it isn't safe, but I think using common sense is the critical thing. I would be hesitant to use Marta late at night when there is limited service and limited riders, but for day time visits to visit Olympic Park, Ga Dome, the Aquarium, MLK sites, and Grant Park, you should be be fine.