This app was mentioned in 3 comments, with an average of 5.67 upvotes
https://itunes.apple.com/us/app/marta/id386648039?mt=8
https://play.google.com/store/apps/details?id=com.itsmarta.martaapp&feature=search_result#?t=W10.
The app has a lot of useful information, but it could be designed a bit better.
In the Android version, there's an icon at the lower right of the screen that looks like a circle with a triangle on it. That's the "nearby" feature, which is a way to determine which routes are near you. From there you can click on the bus icon and find the appropriate route.
I would get a 3, 4, or 7-day MARTA pass depending on how long you're here. This will allow you to ride the transit system as much as you want, without having to pay $2.50 for each trip.
You'll definitely be able to take a train or a bus to all those locations - just be wary about walking around Atlanta at night if you can help it.
If you do get caught out alone at an unsavory hour, please do yourself a favor and call an Uber, Lyft, or a Taxi - particularly after a Braves game, lone tourists can potentially be easy prey for criminals.
We also have a halfway-decent mobile app for our transit system:
Android: https://play.google.com/store/apps/details?id=com.itsmarta.martaapp&hl=en
Apple: https://itunes.apple.com/us/app/marta-on-the-go/id386648039?mt=8
Hope this helps. Mayor Kasim Reed welcomes you to Atlanta!
I can't speak for George, but I am using the official Marta App: https://play.google.com/store/apps/details?id=com.itsmarta.martaapp&hl=en