The ferry from Vila Real de Santo António to Ayamonte on the Spanish side is one of the most enjoyable things to do here, even if you only cross for an hour or two. The boat runs frequently through the day from the terminal on the promenade — small, no-frills, fast. You step off in Spain, hear a different language, see a different architectural tradition, and eat a different cuisine, all within five minutes of leaving Portugal.
Ayamonte is a pretty Andalusian border town with a pleasant old quarter climbing a hill above the ferry terminal. The fish market in the morning is excellent, the tapas bars are cheap and good, and the atmosphere is distinctly Spanish — you can feel the difference from Portugal immediately. A round trip in an afternoon — crossing over for lunch and coming back before dinner — makes for an easy and enjoyable day in two countries.