Colonel Fitzwilliam was interested in talking to Elizabeth at Rosings. What did they talk about?

Col. Fitzwilliam and Elizabeth conversed about Kent, Hertfordshire, books and music. 

He now seated himself by her, and talked so agreeably of Kent and Hertfordshire, of travelling and staying at home, of new books and music, that Elizabeth had never been half so well entertained in that room before; and they conversed with so much spirit and flow, as to draw the attention of Lady Catherine herself, as well as of Mr. Darcy.