I, fore one, might to one few-hour-long club outing, but otherwise I've found it best/easiest to remain at the convention, where there are MORE than enough things during the night hours for those who don't feel like driving elsewhere. There are bars at the hotel and within a block walking distance. The three large convention halls turn into concerts and dance halls (there are a few live music acts ever night of the convention). Free movies and anime showings, parties GALORE and much, much more.
Goth parties, fetish parties, who knows what else.
As for karaoke... Atlanta has plenty of them, but since it's got the whole night-life city thing going, most of the places that do have karaoke do it during weeknights, not weekends.
The places that do do it within 5 miles of the hotel you'll have to call and see what their schedule is.
The Local - (404) 873-5002 - 1.7 miles
Neighbor's Pub - (404) 872-5440 - 2.3 miles
Stooge's - (404) 355-5445 (karaoke Saturdays at 9:30pm) - 3.7 miles
The rest are either bars that people would not want to go to (drive 3 miles in one direction you've got trendy, drive 3 miles in the other direction you've got ghetto). The rest of them (the majority of what could be considered a karaoke bar) are gay/lesbian places.
Driving more than a single car load of people through the city can get to be a nightmare. Not only are you looking at the extra charges for taking cars out of parking garages and putting them back in, you're also stuck looking for more parking when you get there. Best bet is to get a taxi van.