Solo, Jerónimos Monastery (Mosteiro dos Jerónimos) works.
Walk in at opening, before the tour groups, and the two-storey cloister does something the photos can't: the low morning light rakes across the carved stone and the place is genuinely hushed, every column a different sermon in lacework. That quiet half-hour is the actual experience — and it evaporates by mid-morning, when the cloister becomes a shuffling, selfie-stick crush and the awe drains out of it. So the verdict is about timing more than facts: this is the one Belém interior worth a paid ticket and an early alarm. The adjoining church (Vasco da Gama's tomb) is free and most people skip it — don't.