Baldur's Gate: Useful Hidden Stashes
Lion's Way (one area east of Candlekeep)
Diamond
Follow the road leading away from Candlekeep, and right on the left of the trail there will be a tree with a little hole in it's trunk with a diamond in it.
Coast Way (east of Candlekeep, south of the Friendly Arm Inn)
Ring of Protection +1
Near the northern exit to the Friendly Arm Inn, there is a good-sized rock with a hole in it. Inside that hole is this convienent ring which boosts Armor Class and all Saving Throws by 1.
Coordinates: x 1006, y 1114
Friendly Arm Inn
Ring of Wizardry
At the bottom of this map, walk east. There is a tiny part under a tree that contains this ring which doubles the amount of Level 1 spells your mage can have. If you don't have a mage, you can always sell it for a good bit of coin. It will probably take you a good long while to get it, but it's there somewhere.
Coordinates: x 2555, y 3759
NOTE: this is removed in Tales of the Sword Coast, although put back in by the latest TotSC patch.
Nashkel
Ankheg Armor
At the west end of the middle of the map, there is a farmer's field. On the west end of it right below a small tree is a hole that contains this armor and a pearl. The armor is +2 Plate Mail with a lot of the weight cut out, while the pearl isn't good for much other than selling.
Coordinates: x 187, y 2742
NOTE: the popular "Hard Times" mod for BG1 Tutu removes this suit of armor, though the pearl remains.
Fire Leaf Forest (south of Nashkel)
Cloudkill
Sightly below and a bit west of where you enter the map from Nashkel is a small circular ledge area surrounded on three sides by trees. Inside a small gash in the ground is a mighty scroll of Cloudkill for your mage, along with a Chromatic Orb scroll.
Coordinates: x 1793, y 414
Naskhel Mines
Wand of Frost
At the far top left of the map there's a wand of frost hidden in a tree trunk. Good for dealing high damage to a single target, though it's best not used for a finishing blow as it can destroy items.
Coordinates: X 165, Y 180
Gibberling Mountains (east of Nashkel Mines)
Ring of Fire resistance
Approach the cliff in the center of the map from southwest. There's a small crack in the cliff which contains a Ring of Fire Resistance and some gems.
Coordinates: X 1955, Y 2370
Return to Baldur's Gate.