With kids, Praça do Comércio works.
A genuinely magnificent open space — arcaded yellow facades on three sides, an equestrian king in the middle, and the river lapping the marble steps where ships once docked. The catch: it's a vast transit-and-photo zone with little to do but admire it, ringed by touristy cafés. Sunset and night, when it lights up, are when it earns the visit.