lord of the rings - What did Denethor do in the secret room?

I just discovered canon support for DVK's answer that Denethor was looking into his Palantir.

Starting with the words of Gandalf:

"Though the Stewards deemed that it was a secret kept only by themselves, long ago I guessed that here in the White Tower, one at least of the Seven Seeing Stones was preserved. In the days of his wisdom Denethor would not presume to use it to challenge Sauron, knowing the limits of his own strength. But his wisdom failed; and I fear that as the peril of his realm grew he looked in the Stone and was deceived: far too often, I guess, since Boromir departed. He was too great to be subdued to the will of the Dark Power, he saw nonetheless only those things which that Power permitted him to see. The knowledge which he obtained was, doubtless, often of service to him; yet the vision of the great might of Mordor that was shown to him fed the despair of his heart until it overthrew his mind."

"Now I understand what seemed to strange to me!" said Pippin, shuddering at his memories as he spoke. "The Lord went away from the room where Faramir lay; and it was only when he returned that I first thought he was changed, old and broken."

"It was in the very hour that Faramir was brought to the Tower that many of us saw a strange light in the topmost chamber," said Beregron. "But we have seen that light before, and it has long been rumoured in the City that the Lord would at times wrestle in thought with his Enemy."

"Alas! then I have guessed rightly," said Gandalf.
_{The Lord of the Rings Book 5 Chapter 7: The Pyre of Denethor}

And so has DVK; when Denethor withdrew to the secret room under the Tower, he was looking into the Palantir, and in it he saw primarily the forces of Sauron amassing (as well as the ships that had been captured by Aragorn, though Denethor thought they belonged to the Enemy still).