there are a few hidden items I've come across while playing Baldur's gate through-out my life. Such as the ring that is hidden on some rock, in the map directly below Friendly Arm Inn. There was also some armor in the crop field of some city, forget which. Anyways, I'm playing through the game again (first time in 7 years?) and I've forgotten any other locations of items. I know there is something hidden by a tree outside Friendly Arm Inn.
The armor in the Nashkel crop field is ankheg plate mail. Full plate mail is still vastly superior for several reasons.
On the map where Gorion is killed, on the far east road there is a tree with a diamond in it.
On the next map where you meet Elminster for the first time, go north and there will be a rock with a ring of protection+1 in it.
On the Nashkel mines entrance map, there is a tree in the northeastern corner with a wand of frost.
On one of the southeastern maps (the one were you get a quest to deliver a sick man to the FAI temple) along the southern mountain cliffs is a hole with a Ring of Protection from Fire.
If you want detailed coordinates that show exactly where these are, check a FAQ. DSimpson's probably has the coordinates. Also if you use Tutu, the Tab highlight function will highlight these hidden caches as well.
Full plate is indeed superior to ankheg plate unless . . .
. . . .you are using a character too weak to wear full plate and other essential gear and not be encumbered. Viconia comes to mind. Also, her magic resistance more than offsets the inability to use a protection item with this armor.
. . . you are using a single class druid (Faldorn or your PC). The difference between ankheg plate and studded leather or hide is enough to justify sacrificing the save bonus from protection items (which are not allowed with the ankheg plate). I used this on a recent playthrough and it actually made Faldorn worth keeping around for a while.
if you have Throne of bhaal and TuTu installed you can run around with the tab key to highlight all the hidden items. Otherwise you can use cheat keys iirc correctly ctrl-4 to highlight all interactables in the environment (containers, doorways and traps)
I wouldn't recommend such for a first playthrough but if you are just concerned with finding whats around that can show you --- Absinthe makes the heart grow fonder. It didn't work in Mass Effect... There was just too much foreplay and no sex. - Flo in regards to the Bioware formula.
There is also a cache somewhere that has a cloudkill scroll in it, but I can't ever remember where it is; I always have to look it up in an FAQ. --- A good friend will help you move. A really good friend will help you move a body.
The scroll of cloudkill (and a chromatic orb scroll) is in a hidden cache in the map south of nashkel, west of the mines map. Its in a clearing amid some trees in the northwest quadrant of the map, about a quarter of the way down. Not too exciting, but is an easily obtained (and abused) level 5 spell early in the game.
Personally, I almost never use cloudkill - it makes for boring combat (or lack thereof).