Originally Posted by LapLap

https://www.keioplaza.com/facilities/
Unless you are planning to stay there in July or August 2021, the pool (which requires a fee*) won’t be an option.
As the website says, gym would be free for guests over 12 but it is currently off limits.

* pool fees (indoor and outdoor) at Tokyo hotels are common practice, usually somewhere between 2,000 to 3,000yen. Virtually impossible to find an outdoor pool at a hotel which is open before July or after August even when they do admit patrons.