What to see in Porto in 5-6 hours?

I am your age and was in Porto last winter.

In 5 or 6 hours, I think you could see the Train Station where you arrive (tiled), walk down Rua des Flores (flat) , tour the Bolsa (if timing and tour in English work out- it takes about 30 minutes if I recall correctly), see one (golden) church, eat lunch, and enjoy a 55-minute cruise to see the Six Bridges. A guide might help maximize time. Or you could skip a lot of the above and take a 2.5 hour "free" walking tour.

A possible alternative or a day trip for another day is Coimbra, closer to Lisbon. It is an old university with myriad older buildings to see, the most stunning library I've ever seen, a cute little town, yes, some steps downhill, and students in Harry Potter type robes/capes. I had never heard of it and I loved it!! It might be better to take a car/driver to/from Coimbra. We went via train and Bolt hail a ride from Porto, so I don't know.

Any way you choose to do it, enjoy. I was surprised by how much I liked Lisbon (years ago) and, more recently, Porto and Coimbra.

Or on the Gaia side you could

Take the cable car R/T to get an overview,
Tour Sandelman’s to learn about and taste some Port Wine (liked it more than I thought I would).

To get the the Gaia side take a very quick ferry ride.

Church of Sao Francisco. Of all the churches in Europe I've been to, this is one I'll never forget.

There are two train stations in Porto. Sao Bento is the one with the gorgeous tiles.

These are excellent suggestions. The train station I think we will arrive at is Campanha, but I think there are some commuter lines that go to the pretty tiled one. Thank you all very much.