Right behind Perimeter Mall is a MARTA station, which can drop you south into Atlanta quickly, and Uber in the general vicinity. To see anything not too close to MARTA, you will need a car.
Day hike up Stone Mountain is okay. Or ride the cable car up. (Car.)
If you are there Friday 10/21, Friday Night Jazz at The High Museum is fun, if you like that sort of thing. (Arts Center MARTA Station.)
For partying, avoid Buckhead. Elsewhere, you can't throw a rock without hitting a club. Except in Dunwoody. Yelp is you friend.
If you get down to Ponce City Market, you can check out the Beltline and a bunch of restaurants. Vibrant, growing area. (North Avenue MARTA and Uber.)
Check The Variety Playhouse for concerts. They just finished a nice remodel. While there hit Little 5 Points area in general, have a nice dinner, and people watch. Vortex is there. If you are going to spend a night out, make it this. (Inman Park MARTA and Uber.)
I think you are SOOL for Sonoran style Mexican anywhere in Atlanta. Around that area you will have to make do with Chuy's, Tin Lizzy's and Uncle Julio's. All the restaurants around there are chains. No good BBQ in Perimeter either. For not great BBQ near there, Shanes.
Get chili dogs, onion rings, and a Frosted Orange at The Varsity near Georgia Tech. (North Avenue MARTA.)
Late night dessert, coffee, food, check out Cafe Intermezzo in Dunwoody. Excellent. A couple bars near there.