I last did a guided tour of the Douro about 18 months ago on a small group tour. It was a full day adventure, mainly due to the drive in both directions. But most of those in our group also did a 2 hour boat ride in the middle of the day, which they enjoyed. I preferred to spend my time walking the town and having a couple drinks on the river promenade. I can't offer a recommendation for the operator because it was a local guide and arranged by our hotel. It was very nice, 8 people total, and we visited two smaller vineyards and had a very good mid-day meal at a family place off the normal path. Some of the best fish I've had in years.
If you're in Porto, and want a good way to spend an evening I recommend going up to the beach at Matosinhos and having a meal watching the surfers out by the old fort or just walking along the strand while the sun goes down. There are some nice places on the sand for food and drinks.