best way to do day trip to Douro Valley from Porto

Take the Douro Valley train from the Estação de São Bento or Porto Campanhã and save the money. The train is less than 20 euro each way. The train runs along the river on the north bank and you don't have to bother to drive. The view is the same.

The São Bento station is in the city center, but not sure if you have to change trains (depends on the day of the week), but from Porto Campanhã there would be no train changes. I took it from Sao Bento and on that day there were no changes required.

https://www.cp.pt/en

Then when you reach Pinhão, less than 2.5 hours later, if you want a cruise, take a short cruise from there (I think they have them as short as one hour).

When you are done, and had lunch, take the train back.

You can also go by train further up the river to Pocinho and back. I don't know if they have cruises in Pocinho, since I've been only in cruises in Pinhão. Pocinho is probably an extra hour after Pinhão.

Another option is to take one of the many van or bus day tours of the Duoro Valley that often include a one hour boat ride as well as visiting 2 or 3 Quintas for seeing vineyards and tasting plus lunch. Several years ago, I did one a small group ( max 8) van tour with Coo;tour Oporto and liked it very well. Personally, I would not enjoy an hours long Duoro cruise. I believe this tour currently goes for 126 euros per person. Cooltour Oporto probably has some arrangement for private tours but that would be more expensive.