I don't think any of these are particularly hidden so you've probably seen some of them before, but...

Killing Minthara by putting an explosive barrel beside her desk, nobody else noticing (I think they fixed this in a later patch), and getting the popup "Lae'zel is inspired!" Followed, less successfully, by Lae'zel and Shadowheart chucking barrels from the rafters onto the chief in the goblin temple.

Someone saying to me in Rivington, shortly after the boss fight at the end of act 2, "you've crossed <whatever criminal organisation>. Watch your back from now on," to which I could reply (and I paraphrase) "LOL, guess what I just killed," and watching them gulp in terror and back down.

The bard in The Guildhall telling me that this is her spot, and there's no space for another bard. I just joined in with what she was playing (I love that feature), and the people that had basically been ignoring her got up, started cheering and throwing money at me.

Seeing "Zombie Astarion" appear at Cazador's Palace. How many times can he die, and does he come back for revenge as Revenant Zombie Astarion?

Jaheira greeting me in the epilogue with "You can make yourself presentable, when you have a mind to", then the camera turns to my character in probably the most awkward, ridiculous pose of the game
[Linked Image from files.trantion.uk]