You can find out on their web sites. It varies.
It depends on the type of tasting one selects. At many quintas there are several different options--basic, premium, ports only, DOC wines only, tasting with lunch/picnic, tour only, etc.

Quinta do Bomfim has several different tasting options-
https://www.symington.com/visit/quin...omfim/3?step=2

Quinta da Roêda-
https://croftport.com/en/quinta-da-r...isits-tasting/

Quinta das Carvalhas has several different experiences-
https://realcompanhiavelha.pt/en/win...e-experiences/

Quinta de la Rosa-
winery tour, €10; tour & tasting, €20

Quinta Noval-
Private tours only. Our was complimentary.
They have a tasting room at the pier in Pinhão
https://www.quintadonoval.com/en/wine-shops

Quinta do Crasto-
Tour & tasting, €29; plus lunch w/wine, €70

Quinta do Panascal-
Self-guided tour of the vineyards with their audio guide plus tasting, €10
Tasting room is open during certain hours for sampling their ports by the glass

Quinta Nova de Nossa Senhora do Carmo (a Relais & Château property)-
Tour only + museum visit, €16; tasting in their Patamar Petiscaria, starting at €24 and up
They also have a wine bar (no reservations necessary)
https://www.quintanova.com/en/wine-t.../wine-tasting/

Quinta do Tedo-
€14 and up, depending on your selection
https://www.quintadotedo.com/tours-t...-visitors-2022

Quinta do Seixo (Sandeman)
https://www.sandeman.com/port-wine/v...o-seixo-douro/

Quinta da Pacheca--they have a beautiful restaurant and a dozen or so unique accommodations in huge wine barrels set in the vineyards.
https://quintadapacheca.com/pages/pa...ine-experience


Quinta do Pôpa
classic, €20; premium, €25. They also offer picnics
https://quintadopopa.com/en/visite-nos/#prova-de-vinhos

Can you just pop in or are these all by appointment only?

The information about appointments vs the ability to just "pop in" you should find on the websites that I've listed above.

As I mentioned above, Quinta Noval now has a tasting room in Pinhão--no reservations required and again, Quinta Nova has a wine bar that doesn't require advance reservations. The drive up to this stunning estate is beautiful but challenging

And my post above about Quinta do Panascal: "Tasting room is open during certain hours for sampling their ports by the glass"
Check their hours on the web site posted above.

I guess it's the lack of time...If Porto was our first stop, we'd probably spend the extra day going to Guimaraes. But it's our last stop, so we'll probably just take it easy and relax with a nice view and a drink 😉

Hmm, some of these quintas can only be reached by driving narrow, winding, maybe steep roads.

Not the best conditions for sampling wine, perhaps.

That's why it's best to have a designated driver, book a tour or use a taxi service. There is a taxi stand at the Pinhão rail station.
Not all the quintas are that difficult to reach, such as the ones within walking distance or a quick taxi ride from Pinhão or Régua.

Most who have the means to stay at the Quinta Nova, the Relais & Château, mentioned above whose access is challenging, choose to use the hotel as a retreat, a getaway place to relax, to taste wine there, dine there and if they wish an excursion, the quinta books a car/driver for them. This quinta also has its own boat for river cruises.

Or walk, and then stumble back to your hotel. That's what we plan to do:)

There you go, Mel! Stay close and just walk back!:)