Santa Maria degli Angeli was built inside the remains of the Baths of Diocletian, turning part of a vast Roman bath complex into a bright Renaissance church. The unusual setting is the main reason to visit: ancient masonry, later church architecture, and a working place of worship all share the same space.
Inside, look for Francesco Bianchini's meridian line, where the sun marks solar noon. The church is close to Termini and Piazza della Repubblica, so it is easy to fit into a short walk through this part of Rome.