Hah! Came back from there a couple of weeks ago!
One suggestion that comes with a strong recommendation (from me) would be to go to Pinhao. If you are in Porto you can catch the train from either Sao Bento or Campanha station. The line runs up to the Spanish border (nearly). If going from Quinta da Pacheca you can pick up the train from Regua. It’s one of the great railway journeys, and Pinhao’s lovely (but small) station is a tile-decorated joy.
There are several things you could do - an hour’s trip on one of the river boats is interesting as you can observe a number of famous name’s vineyards, and it’s very peaceful and relaxing.
But a 15-minute walk around the corner from Pinhao back in the direction of Regua will bring you to Quinta de la Rosa. They have a good restaurant, and their wines are of a high standard (BBR bring them in to the UK). Their 20-y.o. tawny is I think the best of that category I have tasted. You can have a lovely relaxed meal on their terrace overlooking the Douro, accompanied by their wines, plus a few others that Jorge Moreira (their winemaker) has chosen, including some from his own property, Poeira. Q delaR have a website worth a look if this interests you.
Also worth a look is Symington’s Quinta do Bomfim at the other end of Pinhao. This used to be just a production facility, but they have now built a big reception/tasting area where you can try a lot of their wines and ports, such as Graham’s and Dows. I can particularly recommend the older Dows aged tawnies (30 & 40 years) which to my taste outclass the Grahams ones. Lots to taste here if you have the time. I think you can still book a trip around the production facilities, but you can just turn up for the tasting. If they have room, you may be able to join a tour the same day but I haven’t tried it.
There’s also the Vintage House hotel in Pinhao who have a good restaurant. I seem to recall it’s owned by Taylors or someone like that.