Sahasra Adhi Pura is a Javanese Hindu temple and Kundalini yoga centre on the edge of Solo. It is a newer religious complex rather than an ancient monument, but its miniature representations of sacred sites from around the world make it an unusual stop in a city better known for palaces, batik, and markets.
Come with modest expectations and dress respectfully. It is most interesting if you want to see how Hindu practice survives and adapts in largely Muslim Central Java.