See if you can attend a service at Sinagoga Major. You'll learn about some of the customs and practices of the Jewish faith.
This Jewish house of worship in Downtown Barcelona comes recommended by many travelers, so be sure to put it on your travel itinerary. Don't think the sightseeing ends there, however. There are a lot more spots ready for you to explore after Sinagoga Major. Sagrada Familia, La Rambla, and Plaça de Catalunya are not far away.