Have you been to Japan before? If not, while I won't claim personal familiarity with the MO Tokyo's offerings, instead of paying megabucks for what would presumably be a rather generic 5-star-hotel spa that could be anywhere in the world, I'd suggest visiting an actual Japanese onsen (hot spring) style resort. Eg. Oedo Onsen Monogatari can be reached by direct shuttle bus from Tokyo stn, and continuing from there to Haneda by train or even taxi should be fairly straightforward.