I just recently moved from the Atlanta area -- specifically, Marietta. Kennesaw itself is one of the safest towns in the country. Am I to understand that you are going to tour the Kennesaw Mountain battleground? If so, keep in mind that it is a fairly good hike so dress accordingly. While you are in that area, you might want to check out Marietta Town Square, there's some interesting stuff there.
As far as the Atlanta metro area in general -- my best suggestion is to use your common sense. I'm not all that comfortable riding the MARTA train with the exception of the line that goes from downtown up to the north, which isn't too bad. There are parts of Atlanta that civilized people just don't go, however the touristy parts aren't too bad. If it looks like a bad part of town, it is. In general, the north, northwest (where I lived) and northeast 'burbs are nice and safe. Especially in the downtown area, there's some tourist attractions that aren't far at all from some parts that aren't safe, so do be sure to not stray from the beaten path. If you stay with the other tourists, you ought to be Ok. Clayfish's list of suggestions is pretty good. You might also want to check out Stone Mountain over on the east side.