This site is worth a visit to just view the interior of a Roman-type basilica as a Catholic church. Advertised as "America's First Cathedral," the high, vaulted ceilings, dome, and ornate sanctuary is a visual treat. In the lower levels, there is the crypt which where many of the Archbishops of Baltimore are laid to rest.
Pope John Paul II visited the basilica and there is a beautiful prayer garden bearing his name nearby.
You can do a self-guided tour or with a guide (donations are accepted).