Slums District Overview - Baldur's Gate 2
The Slums is the second location that you'll be visiting in the city of Athkatla, your first time entering this area will begin Chapter 2 of the main story. Upon entering the Slums you'll speak to Gaelan Bayle whose best known as the "Coo!" guy, he'll take you to his house and ask for 20,000G to free Imoen from her prison.
As far as what you can do in the Slums - quite a bit actually. The main spot here worth talking about is Copper Coronet which is a bar in the center of the map. Here you will find a couple of new companions, Nalia, Anomen, Korgan and Hexxat as well as a few early game quests.
Copper Coronet: This bar is going to be one of the most visited zones in the entire game for you. Inside you'll find 2 vendors (one of which sells BiS weapons) and many different companions. Also, many companions return to the Copper Coronet when you dismiss them from your party to wait for you here -- which is another reason you'll constantly be visiting this place. For more information check out my Copper Coronet page.
Night Vendor: You can find a vendor here at night with almost nothing good for sale. Only reason I am pointing him out is he is quite unique and a nice vendor to know about when you're hitting up the temple.
Ployer's House: This building is marked Derelict House on your map and it will be empty until you begin the Save Jaheira from Baron Ployer's Curse side quest. During this side quest you'll find Baron Ployer hiding out inside of this house.
Slaver Stockade: Slaver Stockade is one of the earliest dungeons you can clear in the game. There isn't much to note inside this building aside from the fact that part of the Free Hendak and the Slaves side quest takes place in here.
Gaelan Bayle's House: At the very start of Chapter 2 you will meet Gaelan Bayle, he'll offer to help you free Imoen in exchange for money. On the 2nd floor of this house you'll find Arledrian who is a vendor that will sell a variety of items but most importantly he sells Glasses of Identification which allow you to ID up to 3 items a day.
Jan's House: During Jan's Companion Quest you'll be tasked with visiting his house. He'll leave your party for a little during this quest and wait at this house too.
Planar Sphere: This dungeon will start out locked until you recruit Valygar or kill him and bring his body here. For more information about this dungeon check out my Inside the Planar Sphere quest walkthrough or my Planar Sphere Overview guide.