For nature & scenery, Cascais Day-Trip is hit or miss.
An easy, relaxed seaside escape — a pretty marina, sandy bays, a walkable old town and the scenic train hugging the coast the whole way. The honest catch: it's polished and resort-y rather than 'authentic', beaches are small and packed in summer, and there's no single must-see. The low-stress alternative to a frantic Sintra day.