Change (coins) and Diamonds are no brainers to me. I like what they add, and they are pretty effortless to implement.
I'm also a really big fan of the one that randomizes the building color values each round.
We also generally don't play without the encampments, thief, and Currency exchange. I find these are also relatively simple to implement, which we like.
The most complex one we like to add is the Treasure Chamber. It's not really complicated, but adds an extra scoring step. I like that it adds a new way to get points though.
My thoughts, exactly