Heading across the bridge from Oporto, it is easy to grab a bottle of Port with some olives and pass the day away watching the foot traffic or the boats meander down the River Duoro. With the whole area devoted to Port and the appreciation of it, one can easily be swept away with tons of tastings and pairings, instead of walking and boat watching.
In Gaia, one can easily pick a restaurant or distillery at random and head into the cellars.
Taking the funicular up the steep hill to walk across the bridge to the 16th-century monastery church of Serra do Pilar with its unusual circular cloisters and panoramic views of Oporto and the River Douro is definitely another way to kill sometime.