[Sangetsu]

The "onsen" in Odaiba is little more than a hot tub park. People go there to enjoy the spa treatments for the most part.

Most stays at a ryokan will include private use of the bath for free. But you must reserve the bath in advance. During busy times you can usually get one half-hour use each day. At quieter times you may get an hour.

Or you can do as my GF and I did in July and get a room with an in-room onsen. Many ryokans and hotels are now offering in-room pools, and quite a few are reasonable priced.