Some pro-tips for ya:
Regardless of whether or not you want to keep her in your party, save Neera, add her to your party, take her gem bag, then either drop her or keep her, as you please (she is a good companion, though).
Potions of Master Thievery stack, IIRC.
If you need to quickly lower your reputation to keep your Evil party members from leaving (they'll leave after you hit 18+ (or was it 19+?) reputation, IIRC), murdering someone in a secluded area. If your reputation is high enough before committing the crime and no one else saw you, no one should aggro on you.
Dorn (and his 19 STR) is cool, keep him around. Also, he can he kill some things that are supposed to be invincible (like those ghosts in that one ruin that drop end-game loot) with the Poison Weapon+Explosive Arrow wombo-combo. In BGII:EE, his companion quest gives you the Acorn of Yarrow, which can teleport you from any location in the game (including Hell and Spellhold, IIRC) to a specific tree, where it can then be used again in order to teleport back to a specific location in the overworld. This can let you do some events (like the boat trip to Spellhold) over again.
Bring Viconia with you for your initial meeting with Saraad and on Saraad's companion quest to get some special dialogue options.
Don't forget to click that button with the magnifying glass on it, as it highlights things that you can loot and indicates (most) things that you can interact with. Why it isn't on by default is unknown to me.
There's a Ring of Wizardry in a tree at the south end of the Friendly Arm Inn map.
Don't forget to pickpocket Algernon's Cloak off of Algernon.
Some companions give you special dialogue if they are charmed (by you, I think) and then manually spoken to. For example, Xvar tells you a secret, Kagain wonders why he suddenly has friends, and Dorn threatens to leave the party if you don't stop pulling these kinds of antics on him.
Edwin=win. Don't bring him along to save Neera, though, as he has a special interaction that will resolve the conflict without bloodshed but also make him and Neera mutually exclusive to have in the same party together.
Summon things before difficult fights to make them easier, since your rules will still let you control your characters outside of combat.
Animate Dead scales well in BG:EE, unlike in Icewind Dale:EE.
Don't leave anyone behind in the place that you flood (you know the one). On my first playthrough of the game, Edwin decided to leave my party due to my high reputation inside of there and drowned.
Don't romance Rasaad in the second game/ToB, as the epilogue that you get after doing so is... less than satisfying (unless your headcanon is that you faked the events of it, in which case it becomes cool).
Dorn's companion quest will eventually give you a suit of elven chainmail, IIRC.
You can kill or pickpocket Drizzt when you encounter him in order to take his scimitars (and his suit of +5 mythral chainmail if you killed him). He'll remember that you did so when you encounter him in the second game, though, but you can avoid fighting him (and still get him to help you) if you apologize (and have a good enough reputation, IIRC).
Being an elf named Drizzt with a low reputation score will cause Drizzt to challenge you to the right to his name when you encounter him (I don't remember if this is in the second game, the first game, or both, though).
Nahal's Reckless Dweomer (a spell that only Wild Mages can learn/know) can be used to cast any Mage spell that you know. Yes, even Wish.
There's a config option to reenable the old Ranger/Cleric getting access to the entirety of both the Druid and Cleric spell lists (except for the alignment-restricted spells, of course) bug.
Neera and Viconia both have special dialogue that they give when you first meet Baeloth, IIRC.