What should I wear at sacred sites?
It varies by destination and by site, sometimes within the same city.
Jerusalem. The Old City is conservative across all three faiths. Shoulders and knees covered is the practical minimum — for men and women — at the Western Wall, the Church of the Holy Sepulchre, and the Dome of the Rock compound. Carry a scarf for covering your head in synagogues, in some churches (less strictly enforced), and at the mosques on the Temple Mount. At the Western Wall, men should wear a kippah; there are free ones at the entrance.
Varanasi. At the ghats and the Ganga Aarti, modest dress is expected — shoulders, midriff, and knees covered. Inside the temples, including Kashi Vishwanath, the rules are stricter: remove your shoes (there are lockers at the entrance), no leather items, and no photography at all — phones included. I watched a woman get genuinely upset when a guard confiscated her phone at the door. Don't be that person.
Santiago. The cathedral asks for knees and shoulders to be covered; in practice it's gently enforced during Mass and relaxed the rest of the day. If you're walking the Camino and arriving in hiking gear, change before entering. Bring a sarong or lightweight scarf — it weighs nothing and solves the problem in all three cities.