Peninsula has an excellent plan called Suite Life running at the moment (to 12/09) - 2 nights in a suite for the price of 1. This make a suite basically the same price as a standard room. Did this in May, was excellent.

Rate only available on Pen website and only shows if you try to book a suite for exactly 2 nights. Will be a better deal than any FHR promo unless room size is not important or you need 4pm checkout.

You could add additional nights on a separate booking or change hotels - Four Seasons etc quite close.

Pen is potentially the best in Tokyo at present and benefits from being a stand alone building, not stuck into an office tower like most of the others. PH is getting a bit tired by some reports. RC is brand new and could be interesting as long as you are happy with location in Midtown. It is attached to a massive mall and office complex so plenty to do nearby, the Roppongi complex is also under 10 mins walk away.