Maria and Fred's place is a great place from which to explore Porto. It has amazing views,
is clean, and very comfortable, and is walking distance to many of the cities sights. I'd also recommend walking down the hill to eat at Taberna do Barqueiro, and if you have time to catch the 500 bus along the waterfront to the beach, try surfing if you are inclined. My wife and I and our three sons, had a memorable stay in Porto.