Singapore is hardly a place to expect onsen, but there is a little one up in Sembawang along Gambas Avenue.
It was a fun experience exploring Sembawang Hot Spring Park. Family can have an interactive session there while the elderly can have a nice soaking foot bath too.
Given that the hot spring park is located at a small area beside the air force camp, there isn't any parking space for private vehicles and your best bet is either to park your motor vehicles at Block 114, Yishun Ring Road or take a public bus (to bus stop IDs 57129 / 57121 along Sembawang Road), and then walk over.