Niwa no Yu: A real Tokyo onsen
Koyama 3-25-1, Nerima-ku +81 (0) 3 3990 4126
Nerima-ku's Niwa no Yu Toshimaen, (Toshimaen Garden Spa) is located adjacent to the old amusement park of its namesake, could well be Tokyo's best kept onsen secret.
Niwa no Yu is couple-friendly, with a massive central pool (bathing suit required) and outdoor jacuzzis, all surrounded by picturesque grounds. The outdoor Finnish-style sauna even offers aroma therapy sessions, where you can inhale citrus, lavender and other steamed scents.
The complex has a restaurant that serves traditional fare, recliners with built-in TV sets, massage services and more. There's actually enough to do to make it a full day trip.
For night owls, the 9pm entrance fee at Niwa no Yu is almost half price (¥1,260 yen). Also watch out for periodic discounts and specials.