Well, since several other people have explained Seafarers and Cities & Knights, I won’t bore you with saying the same things. Catan is a great world to play a game in!
Before you spend a lot of money on the expansions, you may want to consider how many people you generally will have play. As Hawkeye alluded to, it’s nearly impossible to play a regular game of Catan with two people. (We’ve tried. It doesn’t work.) So, if players and/or money is a concern, you may want to hold off on the expansions.
I’ll also second that you check out Carcassone. Carcassonne (board game) - Wikipedia