I work downtown so I might be able to give some help.
Allgood tends to stay busy.
Sideways, Buddha bar, Bourbon Street, The baitshack all will be packed with youngins.
Stay away from On the Rocks cause its full of cocky preppies.
Flannigans, Firehouse, and Raw are all pretty good and have downstairs bars. Raw has a hooka bar below it that stays open until 3. So at 2 when raw closes you can walk downstairs with your drinks and chill.
Those are just a handful. Enjoy your birthday and dont get tanked too early. Oh and be careful how you act. Athens has more police per capita than any other city in GA, and they are true assholes.