I don't know about Power grid at all but I recommend Carcassonne. It's immensely fun to build the world and extremely satisfying when you manage to build a large castle. I've only played two expansions which are fairly basic, one introduces a river on the board and the other introduces a Bishop character with an ability to gain points on gardens as well as monasteries.

There's lots if expansions though and they all sound pretty fun. We recently bought Inn's and Cathedrals or something along those lines which Im looking forward to trying. There's a very cool sounding one though involving sieges where you can put broken walls on your enemies castles to reduce their points value.