The circular walk (20km, 6hr) from Miranda to São João das Arribas is the best use of a spare day, though you can also drive there (partly on dirt roads, in good condition) for a picnic. The walk is waymarked with red-yellow paint stripes, and though many are faded they are still all (mostly) visible, making route-finding fairly straightforward. There is also a leaflet available in the park office showing the route; be prepared for barking dogs at farms all the way round.
The route starts by the turismo, where you follow the brown “Castros de Vale de Águia e de Aldeia Nova” sign – the road through the shops and buildings soon turns to a dirt track and the waymarks begin. At Castro de Vale de Águia (2.5km) are the first amazing views over a double bend in the river; at the rustic-in-the-extreme hamlet of Vale de Águia (4.5km) follow the “Aldeia Nova” sign along a tarmac road to Aldeia Nova (6.5km), where there’s a bar (probably closed) and a sign for “São João das Arribas” and “Castro”. Down this track, at the small chapel of São João das Arribas (8km), are simply extraordinary views of the Douro gorge, plus remains of the Iron Age, later Roman, castro. It’s a great place for a picnic.
The no-risk return is to go back the way you came, though the actual waymarked route runs west through Pena Branca (12km) and then south to the Fresno River where you cross the low bridge (16km) and then head up past farms until you crest a hill and see Miranda (20km) ahead.