Hot springs(onsen) are one of the highlights for travellers.
However, you must normally take off your outfit when you go into the baths, especially in authentic hot springs, even a towel is not allowed to cover yourself in the baths.
I guess some people might not feel comfortable being naked in public.
Here is a list of three so-called onsen themed parks which allow you to wear a swimsuit around Tokyo:
Urayasu Mangekyo( Urayasu)
Ooedo-Onsen Monogatari is one of the popular tourist onsen themed parks. Urayasu Mangekyo is one of their branches which has “Swimming wear zone” in it as well as normal hot springs.
As they have seven restaurants and shops inside, you can enjoy strolling around there wearing a colorful Yukata before or after the bath.
Hotel Mikazuki is one of the biggest onsens.
They have an outside swimming pool as well as inside hot springs.
One of their unique features is the Golden Bath, made from gold.
Although I am unsure of the exact therapeutic benefit we are supposed to gain from an experience in this bath.
Hakone Yunessan (Hakone)
Yunessan is another theme park where you can enjoy more than 25 different kinds of onsen.
It is quite well known for its unique onsens such as the Coffee Bath and the Wine Bath.
It suits family trips more than romantic ones.