Just got back yesterday from a week's tour with Riviera.
We visited Lisbon, Sintra, Coimbra, Porto and Obidos, all of which were fascinating and interesting places to spend time. I have to say that Porto is my favourite. Lived near there for three years and the city itself is very interesting with loads to do and see - you could spend days visiting the wine lodges alone. My favourite is Sandeman. Look for the man in the hat and the cloak.
There are great seafood restaurants beside the river, a very striking cathedral, excellent range of shops and markets, and a cable car.
Half an hour on the train going south brings you to the beautiful beaches of Miramar and Aguda. Full of amazing fish restaurants. Can't recommend it enough, and the people are lovely.