Sewers

The grates on the ground in all areas of Baldur's Gate are not just decoration; they are passageways to three areas with connections to the whole of the City, even the Iron Throne and palaces. This would make them a useful shortcut if entering them did not require Disc 4 and exiting, require 5 again -sigh-

West sewers

 * Exit to the basement of the Iron Throne at the northwest end
 * Schlumpsha the Sewer King and crew -2235, 2065. The primary, and in all other ways excellent guides for Baldur's Gate do not merely advocate but recommend killing in a lot of the circumstances that do not incur Reputation loss, where they quite rightly shy away from telling the player how to play the game in other ways, such as stealing. There is no shortage of combat XP in BG1, and much of the XP comes from quests in any case, so this just seems bloodthirsty. Schlumpsha the Sewer King and his motley collection of untouchables are reminiscent of the Ghouls in the Fallout series, who are neutral if not antagonized. Left alone, they add a bit of color, with their shambling and sound files, to an otherwise grey dungeon crawl in the Sewers.

Central sewers
Exit to the Duchal Palace in the west, second passage from the top
 * Ratchild -2305, 2090, with his party of Kobold Commandos, hails the party and then attacks, just like any surface highwayman group. Because he engages the party in conversation, the expedient strategy of attacking with ranged is compromised, so make sure to send a character ahead with Fire protection on them (Protection from Fire scroll, Potion of Fire Resistance, Cleric spell Resist Fire and Cold spell, all of them +50%. Ring of Fire Resistance +40% from West of Nashkel, Cleric and Druid Protection from Fire spell +80%). 350 XP, twenty Arrows of Fire

East sewers
AR 0226 Ogre Mage and Carrion Crawler pets at 1335, 1590. The target of a quest from the Flaming Fist's Scar. Bastard Sword +1, Ruby Ring (return the ring to Scar; obviously do not sell it, but also, Farmer Brun in North of the Friendly Arm would take any Ruby Rings should the player return his son's body. Should the worst come to the worst, Scar will still complete the quest without the ring, but give a lesser reward. Or find another ring; there is one in Durlag's Tower)

Undercellars
Assassins Slythe and Krystin engage in dialogue at the center of the crossroads of the undercellars. 8000 XP, Invitation to Coronation, Letter, Dagger +2, Sling +1, Short Sword of Backstabbing, ten Bullets +1, ten Bullets +2, Potion of Master Thievery, third level spell scrolls Clairvoyance, Flame Arrow, Non-Detection, Protection from Normal Missiles, Skull Trap, Vampiric Touch, and fourth level spell scroll Monster Summoning I, and 132 GP