What expansions for Carcassonne add variety without changing the character too much?
Carcassonne: Traders &amp; Builders 

The chance with the builder to draw two tiles can make a huge impact on the game so it adds a dimension all on it's own. I however personally prefer not playing with the Traders part (the cloth, wine and grain tokens) just the builder and the tiles.

Carcassonne: Bridges, Castles, and Bazaars

Bridges makes for some interesting escapes and castles is a new fun thing to do with those small cities. As above I preferably skip parts of this expansion which is Bazaars (tile auction).

Carcassonne: Inns &amp; Cathedrals

Pretty straight forward expansion that adds Inns (a road with an inn somewhere along it's path is worth 2 points when complete (0 if left unfinished) per tile. Cathedrals are in cities and makes the city tiles worth 3 points if completed (0 if not completed) per tile. Great ways to remove others points if drawn late in the game. Also the big meeple/guy is in here iirc.

Carcassonne: The River II 

This one, unlike first expansion can split and create two streams.

Finally I just recently started added Carcassonne: The Cult to my bag. Works just like cloisters but if you put it next to a cloister only the one scoring first gets any points. So far it's fun but it's only some 5 tiles or so.