If you can, skip the packed touristboat tours & hire a private boat for the day. It's not as pricey as you'd think..Stunning views + a welcome change of pace from climbing the steep cobbled streets.
Swimming off the back of a FeelDouro boat on the Douro River--a fab, leisurely private tour of the river/Portuguese countryside that you can even at very short notice--we only booked a day in advance.