Laughing Skull is not a bad idea, but double-check the calendar at The Punch Line before you decide for sure. The Punch Line is probably the city's best-known: http://www.punchline.com/index.asp
As for where to eat along the way and while you're here, I hope that you'll bookmark the Marie, Let's Eat! map: http://www.communitywalk.com/marie_lets_eat/map/939046
Honestly, Mary Mac's is feeling its age and getting a little more touristy with every passing year. It's nowhere as dire as what's become of Pittypat's Porch, mind.
As for museums, the High Museum of Art is terrific, and so is the Atlanta History Center, Fernbank, the Atlanta Botanical Garden, and the Center for Puppetry Arts, and each of these places has very good restaurants nearby. Fernbank, in particular, you're about five minutes' drive from Decatur and you'll have no end of great choices for something to eat, like Chai Pani, Taqueria del Sol, Brick Store Pub, Raging Burrito, and Victory Sandwich Bar.