I say, dear Bombadil...

[Kalimac]

Probably about anything and everything except the Ring and what went with it. After the last year and half, Gandalf was probably just about Middle-Earthed, Sauron'd and Fate-Of-World'd out. He and Tom could trade stories of the old days, go walking in the woods and eat good food, and generally take a long breather. Sort of like the day after you take the board exams; the last thing you want to do is rehash all the studying you did and what the questions were. All you know is that it's over, you're fine, and it's time to rest.