Hi, ausmarko,
Margaret is right about the beauty of the Douro Valley. I have been there in winter, near harvest time, and also in the early summer, and it is always breathtaking. I am assuming you will not have a car here, so you will be limited somewhat by the availability of public transportation. There is good train service up and down the river, so I would suggest you take a train either to Peso da Regua (called simply Regua in the area) or Pinhao. Both are on the river. Regua is a much bigger place, has a lot of commerce and isn't really terribly attractive but is well located and has good restaurants. Pinhao is a much smaller, sleepier little town and has some budget accommodations.
Without a car, you are really not going to be able to go up and weave around the vineyards overlooking the river or back away from the river. If I'm wrong about the car, let me know and I can suggest some nice driving routes to visit vineyards, many of them nestled away near beautiful little villages.
If you don't have a car, I think train is your best bet. I would HIGHLY recommend a train trip from either Regua or Pinhao eastward to the end of the line, a sleepy little town called Pocinho. It's about a 45 minute train ride from Pinhao (rough estimate based on fading memory). It's a little chug chug train, very local and weaves along the river in a stretch where there are no roads, just incredible views. There's about a half hour stop at the end of the line while the crew has a coffee in the bar, then the return trip takes about the same amount of time. The Pinhao to Pocinho section is the most beautiful by far. I will post some pictures when I'm at a computer that allows me to do that.
If you do have a car, I'd suggest spending the night in Mesao Frio. There's a moderately priced B&B I like a lot, Casa das Canilhas where you will have a balcony or patio just looking down over the river below. It's beautiful.
So this is good for a start, but let me know about transportation, how many days, and budget, and I can probably ramble on a bit more. Buen camino, Laurie