Tokyo’s most famous onsen is undoubtedly Oedo Onsen Monogatari in Odaiba. Its attraction is a combination of easy access, a multitude of services, and Edo-style amusement arcade designed with young people and foreigners in mind (how many other onsen have an area where you can throw ninja stars?).
Despite the veneer of artificialness, it is a natural onsen with the water pumped up from 1,400 meters underground. There is also a “Doctor Fish” foot bath where you sit with your legs dangling in the water and let small fish nibble at the dead skin on your feet.