We stayed in an air bnb a bit out of town, but on the tramline, which only took 10 minutes into town, there are lots of suburbs with cool bars and restaurants all along the way.
Eating near the opera house or right on the water will cost double, as you would probably expect.
On the South side of Douro bridge is a bit of a hill overlooking the river, always seems to be full of backpackers and young locals having a beer and watching the afternoon fade away to sunset (locals sell you beer and food from eskies which is pretty cool)
There is always someone playing live music too, it's a really good vibe, and lots of bars & cafes on the main drag next to it if you need to get out the sun for a bit.
Would also recommend doing one of the free walking tours so you can at least see the sights while you are there (expected tip is about €20 per person, but still great value)